diff --git a/pkg/dart2wasm/lib/class_info.dart b/pkg/dart2wasm/lib/class_info.dart
index 44650e67d70..b683be712ab 100644
--- a/pkg/dart2wasm/lib/class_info.dart
+++ b/pkg/dart2wasm/lib/class_info.dart
@@ -7,7 +7,6 @@ import 'dart:math';
 import 'package:dart2wasm/translator.dart';
 
 import 'package:kernel/ast.dart';
-import 'package:kernel/library_index.dart';
 
 import 'package:wasm_builder/wasm_builder.dart' as w;
 
@@ -217,25 +216,25 @@ class ClassInfoCollector {
   /// shape class with that many fields.
   final Map<int, w.StructType> _recordStructs = {};
 
-  /// Masquerades for implementation classes. For each entry of the map, all
-  /// subtypes of the key masquerade as the value.
-  late final Map<Class, Class> _masquerades = _computeMasquerades();
+  /// Any subtype of these needs to masqueraded (modulo special js-compatibility
+  /// mode semantics) or specially treated due to being from a different type
+  /// (e.g. record, closure)
+  late final Set<Class> masqueraded = _computeMasquerades();
 
-  /// Masqueraded types are mapped to these classes.
-  late final Set<Class> masqueradeValues = _masquerades.values.toSet();
-
-  Map<Class, Class> _computeMasquerades() {
-    final map = {
-      translator.coreTypes.boolClass: translator.coreTypes.boolClass,
-      translator.coreTypes.intClass: translator.coreTypes.intClass,
-      translator.coreTypes.doubleClass: translator.coreTypes.doubleClass,
-      translator.coreTypes.stringClass: translator.coreTypes.stringClass,
-      translator.index.getClass("dart:core", "_Type"):
-          translator.coreTypes.typeClass,
-      translator.index.getClass("dart:core", "_ListBase"):
-          translator.coreTypes.listClass
+  Set<Class> _computeMasquerades() {
+    final values = {
+      translator.coreTypes.boolClass,
+      translator.coreTypes.intClass,
+      translator.coreTypes.doubleClass,
+      translator.coreTypes.stringClass,
+      translator.coreTypes.functionClass,
+      translator.coreTypes.recordClass,
+      translator.index.getClass("dart:core", "_Type"),
+      translator.index.getClass("dart:core", "_ListBase"),
     };
     for (final name in const <String>[
+      "ByteBuffer",
+      "ByteData",
       "Int8List",
       "Uint8List",
       "Uint8ClampedList",
@@ -252,49 +251,9 @@ class ClassInfoCollector {
       "Float64x2List",
     ]) {
       final Class? cls = translator.index.tryGetClass("dart:typed_data", name);
-      if (cls != null) {
-        map[cls] = cls;
-      }
+      if (cls != null) values.add(cls);
     }
-    return map;
-  }
-
-  late final Set<Class> _neverMasquerades = _computeNeverMasquerades();
-
-  /// These types switch from properly reified non-masquerading types in regular
-  /// Dart2Wasm mode to masquerading types in js compatibility mode.
-  final Set<String> jsCompatibilityTypes = {
-    "JSStringImpl",
-    "JSArrayBufferImpl",
-    "JSDataViewImpl",
-    "JSInt8ArrayImpl",
-    "JSUint8ArrayImpl",
-    "JSUint8ClampedArrayImpl",
-    "JSInt16ArrayImpl",
-    "JSUint16ArrayImpl",
-    "JSInt32ArrayImpl",
-    "JSInt32x4ArrayImpl",
-    "JSUint32ArrayImpl",
-    "JSBigUint64ArrayImpl",
-    "JSBigInt64ArrayImpl",
-    "JSFloat32ArrayImpl",
-    "JSFloat32x4ArrayImpl",
-    "JSFloat64ArrayImpl",
-    "JSFloat64x2ArrayImpl",
-  };
-
-  Set<Class> _computeNeverMasquerades() {
-    // The JS types do not masquerade in regular Dart2Wasm, but they aren't
-    // always used so we have to construct this set programmatically.
-    final jsTypesLibraryIndex =
-        LibraryIndex(translator.component, ["dart:_js_types"]);
-    final neverMasquerades = [
-      if (!translator.options.jsCompatibility) ...jsCompatibilityTypes,
-    ]
-        .map((name) => jsTypesLibraryIndex.tryGetClass("dart:_js_types", name))
-        .toSet();
-    neverMasquerades.removeWhere((c) => c == null);
-    return neverMasquerades.cast<Class>();
+    return values;
   }
 
   /// Wasm field type for fields with type [_Type]. Fields of this type are
@@ -341,14 +300,14 @@ class ClassInfoCollector {
       }
 
       // In the Wasm type hierarchy, Object, bool and num sit directly below
-      // the Top type. The implementation classes _StringBase and _Type sit
+      // the Top type. The implementation classes WasmStringBase and _Type sit
       // directly below the public classes they implement.
       // All other classes sit below their superclass.
       ClassInfo superInfo = cls == translator.coreTypes.boolClass ||
               cls == translator.coreTypes.numClass
           ? topInfo
           : (!translator.options.jsCompatibility &&
-                      cls == translator.stringBaseClass) ||
+                      cls == translator.wasmStringBaseClass) ||
                   cls == translator.typeClass
               ? translator.classInfo[cls.implementedTypes.single.classNode]!
               : translator.classInfo[superclass]!;
@@ -390,29 +349,6 @@ class ClassInfoCollector {
     translator.classes[classId] = info;
     translator.classInfo[cls] = info;
     translator.classForHeapType.putIfAbsent(info.struct, () => info!);
-
-    ClassInfo? computeMasquerade() {
-      if (_neverMasquerades.contains(cls)) {
-        return null;
-      }
-      if (info!.superInfo?.masquerade != null) {
-        return info.superInfo!.masquerade;
-      }
-      for (Supertype implemented in cls.implementedTypes) {
-        ClassInfo? implementedMasquerade =
-            translator.classInfo[implemented.classNode]!.masquerade;
-        if (implementedMasquerade != null) {
-          return implementedMasquerade;
-        }
-      }
-      Class? selfMasquerade = _masquerades[cls];
-      if (selfMasquerade != null) {
-        return translator.classInfo[selfMasquerade]!;
-      }
-      return null;
-    }
-
-    info.masquerade = computeMasquerade();
   }
 
   void _createStructForRecordClass(Map<Class, int> classIds, Class cls) {
@@ -505,7 +441,7 @@ class ClassInfoCollector {
     const int firstClassId = 1;
 
     translator.classIdNumbering =
-        ClassIdNumbering._number(translator, masqueradeValues, firstClassId);
+        ClassIdNumbering._number(translator, masqueraded, firstClassId);
     final classIds = translator.classIdNumbering.classIds;
     final dfsOrder = translator.classIdNumbering.dfsOrder;
 
@@ -560,12 +496,18 @@ class ClassIdNumbering {
   final Map<Class, List<Class>> _subclasses;
   final Map<Class, List<Class>> _implementors;
   final List<Range> _concreteSubclassIdRanges;
+  final Set<Class> _masqueraded;
 
   final List<Class> dfsOrder;
   final Map<Class, int> classIds;
 
-  ClassIdNumbering._(this._subclasses, this._implementors,
-      this._concreteSubclassIdRanges, this.dfsOrder, this.classIds);
+  ClassIdNumbering._(
+      this._subclasses,
+      this._implementors,
+      this._concreteSubclassIdRanges,
+      this._masqueraded,
+      this.dfsOrder,
+      this.classIds);
 
   final Map<Class, Set<Class>> _transitiveImplementors = {};
   Set<Class> _getTransitiveImplementors(Class klass) {
@@ -611,8 +553,19 @@ class ClassIdNumbering {
     return _concreteClassIdRanges[klass] = ranges;
   }
 
+  late final int firstNonMasqueradedInterfaceClassCid = (() {
+    int lastMasqueradedClassId = 0;
+    for (final cls in _masqueraded) {
+      final ranges = getConcreteClassIdRanges(cls);
+      if (ranges.isNotEmpty) {
+        lastMasqueradedClassId = max(lastMasqueradedClassId, ranges.last.end);
+      }
+    }
+    return lastMasqueradedClassId + 1;
+  })();
+
   static ClassIdNumbering _number(
-      Translator translator, Set<Class> masqueradeValues, int firstClassId) {
+      Translator translator, Set<Class> masqueraded, int firstClassId) {
     // Make graph from class to its subclasses.
     late final Class root;
     final subclasses = <Class, List<Class>>{};
@@ -645,7 +598,8 @@ class ClassIdNumbering {
     final fixedOrder = <Class, int>{
       translator.coreTypes.boolClass: -10,
       translator.coreTypes.numClass: -9,
-      if (!translator.options.jsCompatibility) translator.stringBaseClass: -8,
+      if (!translator.options.jsCompatibility)
+        translator.wasmStringBaseClass: -8,
       translator.jsStringClass: -7,
       translator.typeClass: -6,
       translator.listBaseClass: -5,
@@ -657,7 +611,7 @@ class ClassIdNumbering {
 
       final importUri = klass.enclosingLibrary.importUri.toString();
       if (importUri.startsWith('dart:')) {
-        if (masqueradeValues.contains(klass)) return -1;
+        if (masqueraded.contains(klass)) return -1;
         // Bundle the typed data and collection together, they may not have
         // common base class except for `Object` but most of them have similar
         // selectors.
@@ -700,26 +654,8 @@ class ClassIdNumbering {
         firstClassId + classCount, Range.empty(),
         growable: false);
 
-    // TODO: We may consider removing the type category table. But until we do
-    // we have to make sure that all masqueraded types that concrete classes may
-    // be mapped to have class ids that are <=255.
-    //
-    // We still want to maintain that all classes's concrete subclasses form a
-    // single continious class-id range.
-    //
-    // => So we move the abstract masqeraded classes before all the concrete
-    // ones.
-    int nextAbstractClassId = firstClassId;
-    for (Class cls in masqueradeValues) {
-      if (cls.isAbstract) {
-        assert(classIds[cls] == null);
-        classIds[cls] = nextAbstractClassId++;
-      }
-    }
-    int nextConcreteClassId = nextAbstractClassId;
-    nextAbstractClassId = firstClassId +
-        (nextAbstractClassId - firstClassId) +
-        concreteClassCount;
+    int nextConcreteClassId = firstClassId;
+    int nextAbstractClassId = firstClassId + concreteClassCount;
     dfs(root, (Class cls) {
       dfsOrder.add(cls);
       if (cls.isAbstract) {
@@ -740,11 +676,9 @@ class ClassIdNumbering {
         concreteSubclassRanges[classIds[cls]!] = range;
       }
     });
-    for (Class cls in masqueradeValues) {
-      assert(classIds[cls]! <= 255);
-    }
-    return ClassIdNumbering._(
-        subclasses, implementors, concreteSubclassRanges, dfsOrder, classIds);
+
+    return ClassIdNumbering._(subclasses, implementors, concreteSubclassRanges,
+        masqueraded, dfsOrder, classIds);
   }
 }
 
diff --git a/pkg/dart2wasm/lib/code_generator.dart b/pkg/dart2wasm/lib/code_generator.dart
index 509ff61137e..6d2e8a42931 100644
--- a/pkg/dart2wasm/lib/code_generator.dart
+++ b/pkg/dart2wasm/lib/code_generator.dart
@@ -3803,3 +3803,46 @@ enum _VirtualCallKind {
 
   bool get isSetter => this == _VirtualCallKind.Set;
 }
+
+extension MacroAssembler on w.InstructionsBuilder {
+  // Expects there to be a i32 on the stack, will consume it and leave
+  // true/false on the stack.
+  void emitClassIdRangeCheck(CodeGenerator codeGen, List<Range> ranges) {
+    if (ranges.isEmpty) {
+      drop();
+      i32_const(0);
+    } else if (ranges.length == 1) {
+      final range = ranges[0];
+
+      i32_const(range.start);
+      if (range.length == 1) {
+        i32_eq();
+      } else {
+        i32_sub();
+        i32_const(range.length);
+        i32_lt_u();
+      }
+    } else {
+      w.Local idLocal = codeGen.addLocal(w.NumType.i32);
+      local_set(idLocal);
+      w.Label done = block(const [], const [w.NumType.i32]);
+      i32_const(1);
+
+      for (Range range in ranges) {
+        local_get(idLocal);
+        i32_const(range.start);
+        if (range.length == 1) {
+          i32_eq();
+        } else {
+          i32_sub();
+          i32_const(range.length);
+          i32_lt_u();
+        }
+        br_if(done);
+      }
+      drop();
+      i32_const(0);
+      end(); // done
+    }
+  }
+}
diff --git a/pkg/dart2wasm/lib/compiler_options.dart b/pkg/dart2wasm/lib/compiler_options.dart
index f7377f7f25c..992d55ef2c7 100644
--- a/pkg/dart2wasm/lib/compiler_options.dart
+++ b/pkg/dart2wasm/lib/compiler_options.dart
@@ -18,7 +18,7 @@ class WasmCompilerOptions {
   String outputFile;
   String? depFile;
   String? outputJSRuntimeFile;
-  Map<String, String> environment = const {};
+  Map<String, String> environment = {};
   Map<fe.ExperimentalFlag, bool> feExperimentalFlags = const {};
   String? multiRootScheme;
   List<Uri> multiRoots = const [];
diff --git a/pkg/dart2wasm/lib/dart2wasm.dart b/pkg/dart2wasm/lib/dart2wasm.dart
index 7fb8339d41a..2c9d8c1173c 100644
--- a/pkg/dart2wasm/lib/dart2wasm.dart
+++ b/pkg/dart2wasm/lib/dart2wasm.dart
@@ -36,9 +36,10 @@ final List<Option> options = [
       defaultsTo: _d.translatorOptions.printKernel),
   Flag("print-wasm", (o, value) => o.translatorOptions.printWasm = value,
       defaultsTo: _d.translatorOptions.printWasm),
-  Flag("js-compatibility",
-      (o, value) => o.translatorOptions.jsCompatibility = value,
-      defaultsTo: _d.translatorOptions.jsCompatibility),
+  Flag("js-compatibility", (o, value) {
+    o.translatorOptions.jsCompatibility = value;
+    o.environment['dart.wasm.js_compatibility'] = 'true';
+  }, defaultsTo: _d.translatorOptions.jsCompatibility),
   Flag(
       "enable-asserts", (o, value) => o.translatorOptions.enableAsserts = value,
       defaultsTo: _d.translatorOptions.enableAsserts),
@@ -73,7 +74,7 @@ final List<Option> options = [
   IntMultiOption(
       "watch", (o, values) => o.translatorOptions.watchPoints = values),
   StringMultiOption(
-      "define", (o, values) => o.environment = processEnvironment(values),
+      "define", (o, values) => o.environment.addAll(processEnvironment(values)),
       abbr: "D"),
   StringMultiOption(
       "enable-experiment",
diff --git a/pkg/dart2wasm/lib/intrinsics.dart b/pkg/dart2wasm/lib/intrinsics.dart
index 0e7006dd563..dbe9ac10f3b 100644
--- a/pkg/dart2wasm/lib/intrinsics.dart
+++ b/pkg/dart2wasm/lib/intrinsics.dart
@@ -404,24 +404,35 @@ class Intrinsifier {
   /// Generate inline code for a [StaticGet] if the member is an inlined
   /// intrinsic.
   w.ValueType? generateStaticGetterIntrinsic(StaticGet node) {
-    Member target = node.target;
+    final Member target = node.target;
+    final Class? cls = target.enclosingClass;
 
     // ClassID getters
-    String? libAndClassName = translator.getPragma(target, "wasm:class-id");
-    if (libAndClassName != null) {
-      List<String> libAndClassNameParts = libAndClassName.split("#");
-      final String lib = libAndClassNameParts[0];
-      final String className = libAndClassNameParts[1];
-      Class cls = translator.libraries
-          .firstWhere((l) => l.name == lib && l.importUri.scheme == 'dart',
-              orElse: () => throw 'Library $lib not found (${target.location})')
-          .classes
-          .firstWhere((c) => c.name == className,
-              orElse: () => throw 'Class $className not found in library $lib '
-                  '(${target.location})');
-      int classId = translator.classInfo[cls]!.classId;
-      b.i64_const(classId);
-      return w.NumType.i64;
+    if (cls?.name == 'ClassID') {
+      final libAndClassName = translator.getPragma(target, "wasm:class-id");
+      if (libAndClassName != null) {
+        List<String> libAndClassNameParts = libAndClassName.split("#");
+        final String lib = libAndClassNameParts[0];
+        final String className = libAndClassNameParts[1];
+        Class cls = translator.libraries
+            .firstWhere((l) => l.name == lib && l.importUri.scheme == 'dart',
+                orElse: () =>
+                    throw 'Library $lib not found (${target.location})')
+            .classes
+            .firstWhere((c) => c.name == className,
+                orElse: () =>
+                    throw 'Class $className not found in library $lib '
+                        '(${target.location})');
+        int classId = translator.classInfo[cls]!.classId;
+        b.i64_const(classId);
+        return w.NumType.i64;
+      }
+
+      if (target.name.text == 'firstNonMasqueradedInterfaceClassCid') {
+        b.i64_const(
+            translator.classIdNumbering.firstNonMasqueradedInterfaceClassCid);
+        return w.NumType.i64;
+      }
     }
 
     // nullptr
@@ -440,35 +451,6 @@ class Intrinsifier {
         b.i32_const(0);
         return w.NumType.i32;
       }
-      if (node.target.enclosingLibrary == translator.coreTypes.coreLibrary) {
-        switch (node.target.name.text) {
-          case "_typeCategoryAbstractClass":
-            translator.types.typeCategoryTable;
-            b.i32_const(translator.types.typeCategoryAbstractClass);
-            return w.NumType.i32;
-          case "_typeCategoryObject":
-            translator.types.typeCategoryTable;
-            b.i32_const(translator.types.typeCategoryObject);
-            return w.NumType.i32;
-          case "_typeCategoryFunction":
-            translator.types.typeCategoryTable;
-            b.i32_const(translator.types.typeCategoryFunction);
-            return w.NumType.i32;
-          case "_typeCategoryRecord":
-            translator.types.typeCategoryTable;
-            b.i32_const(translator.types.typeCategoryRecord);
-            return w.NumType.i32;
-          case "_typeCategoryNotMasqueraded":
-            translator.types.typeCategoryTable;
-            b.i32_const(translator.types.typeCategoryNotMasqueraded);
-            return w.NumType.i32;
-
-          case "_typeCategoryTable":
-            translator.types.typeCategoryTable;
-            b.global_get(translator.types.typeCategoryTable);
-            return translator.types.typeCategoryTable.type.type;
-        }
-      }
     }
 
     return null;
@@ -659,16 +641,38 @@ class Intrinsifier {
           return translator.types.makeTypeRulesSubstitutions(b);
         case "_getTypeNames":
           return translator.types.makeTypeNames(b);
-        case "_isRecordInstance":
-          Expression o = node.arguments.positional.single;
-          b.global_get(translator.types.typeCategoryTable);
-          codeGen.wrap(o, translator.topInfo.nonNullableType);
-          b.struct_get(translator.topInfo.struct, FieldIndex.classId);
-          b.array_get_u(
-              (translator.types.typeCategoryTable.type.type as w.RefType)
-                  .heapType as w.ArrayType);
-          b.i32_const(translator.types.typeCategoryRecord);
-          b.i32_eq();
+        case "_isObjectClassId":
+          final classId = node.arguments.positional.single;
+
+          final objectClassId = translator
+              .classIdNumbering.classIds[translator.coreTypes.objectClass]!;
+
+          codeGen.wrap(classId, w.NumType.i64);
+          b.i32_wrap_i64();
+          b.emitClassIdRangeCheck(
+              codeGen, [Range(objectClassId, objectClassId)]);
+          return w.NumType.i32;
+        case "_isClosureClassId":
+          final classId = node.arguments.positional.single;
+
+          final ranges = translator.classIdNumbering
+              .getConcreteClassIdRanges(translator.coreTypes.functionClass);
+          assert(ranges.length <= 1);
+
+          codeGen.wrap(classId, w.NumType.i64);
+          b.i32_wrap_i64();
+          b.emitClassIdRangeCheck(codeGen, ranges);
+          return w.NumType.i32;
+        case "_isRecordClassId":
+          final classId = node.arguments.positional.single;
+
+          final ranges = translator.classIdNumbering
+              .getConcreteClassIdRanges(translator.coreTypes.recordClass);
+          assert(ranges.length <= 1);
+
+          codeGen.wrap(classId, w.NumType.i64);
+          b.i32_wrap_i64();
+          b.emitClassIdRangeCheck(codeGen, ranges);
           return w.NumType.i32;
       }
     }
diff --git a/pkg/dart2wasm/lib/kernel_nodes.dart b/pkg/dart2wasm/lib/kernel_nodes.dart
index c519a8a3fd0..69d1dad1ea5 100644
--- a/pkg/dart2wasm/lib/kernel_nodes.dart
+++ b/pkg/dart2wasm/lib/kernel_nodes.dart
@@ -50,6 +50,8 @@ mixin KernelNodes {
       index.getClass("dart:core", "_GrowableList");
   late final Class immutableListClass =
       index.getClass("dart:core", "_ImmutableList");
+  late final Class wasmStringBaseClass =
+      index.getClass("dart:_internal", "WasmStringBase");
   late final Class stringBaseClass =
       index.getClass("dart:_string", "StringBase");
   late final Class oneByteStringClass =
diff --git a/pkg/dart2wasm/lib/types.dart b/pkg/dart2wasm/lib/types.dart
index a9f040df61b..fe174e5f573 100644
--- a/pkg/dart2wasm/lib/types.dart
+++ b/pkg/dart2wasm/lib/types.dart
@@ -3,7 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:math' show max;
-import 'dart:typed_data' show Uint8List;
 
 import 'package:dart2wasm/class_info.dart';
 import 'package:dart2wasm/code_generator.dart';
@@ -101,9 +100,6 @@ class Types {
   /// parameter index range of their corresponding function type.
   Map<StructuralParameter, int> functionTypeParameterIndex = Map.identity();
 
-  /// An `i8` array of type category values, indexed by class ID.
-  late final w.Global typeCategoryTable = _buildTypeCategoryTable();
-
   Types(this.translator);
 
   w.ValueType classAndFieldToType(Class cls, int fieldIndex) =>
@@ -288,77 +284,6 @@ class Types {
     return typeNamesType;
   }
 
-  // None of these integers belong to masqueraded types like Uint8List.
-  late final int typeCategoryAbstractClass;
-  late final int typeCategoryObject;
-  late final int typeCategoryFunction;
-  late final int typeCategoryRecord;
-  late final int typeCategoryNotMasqueraded;
-
-  /// Build a global array of byte values used to categorize runtime types.
-  w.Global _buildTypeCategoryTable() {
-    // Find 5 class ids whose classes are not masqueraded.
-    final typeCategories = <int>[];
-    for (int i = 0; i < translator.classes.length; i++) {
-      final info = translator.classes[i];
-      if (!translator.classInfoCollector.masqueradeValues.contains(info.cls)) {
-        typeCategories.add(i);
-        if (typeCategories.length == 5) break;
-      }
-    }
-    assert(typeCategories.length == 5 && typeCategories.last <= 255);
-    typeCategoryAbstractClass = typeCategories[0];
-    typeCategoryObject = typeCategories[1];
-    typeCategoryFunction = typeCategories[2];
-    typeCategoryRecord = typeCategories[3];
-    typeCategoryNotMasqueraded = typeCategories[4];
-
-    Set<Class> recordClasses = Set.from(translator.recordClasses.values);
-    Uint8List table = Uint8List(translator.classes.length);
-    for (int i = 0; i < translator.classes.length; i++) {
-      ClassInfo info = translator.classes[i];
-      ClassInfo? masquerade = info.masquerade;
-      Class? cls = info.cls;
-      int category;
-      if (cls == null || cls.isAbstract) {
-        category = typeCategoryAbstractClass;
-      } else if (cls == coreTypes.objectClass) {
-        category = typeCategoryObject;
-      } else if (cls == translator.closureClass) {
-        category = typeCategoryFunction;
-      } else if (recordClasses.contains(cls)) {
-        category = typeCategoryRecord;
-      } else if (masquerade == null || masquerade.classId == i) {
-        category = typeCategoryNotMasqueraded;
-      } else {
-        // Masqueraded class
-        assert(cls.enclosingLibrary.importUri.scheme == "dart");
-        assert(!typeCategories.contains(masquerade.classId));
-        assert(masquerade.classId <= 255);
-        category = masquerade.classId;
-      }
-      table[i] = category;
-    }
-
-    final segment = translator.m.dataSegments.define(table);
-    w.ArrayType arrayType = translator.arrayTypeForDartType(
-        InterfaceType(translator.wasmI8Class, Nullability.nonNullable));
-    final global = translator.m.globals
-        .define(w.GlobalType(w.RefType.def(arrayType, nullable: false)));
-    // Initialize the global to a dummy array, since `array.new_data` is not
-    // a constant instruction and thus can't be used in the initializer.
-    global.initializer.array_new_fixed(arrayType, 0);
-    global.initializer.end();
-    // Create the actual table in the init function.
-    final b = translator.initFunction.body;
-    b.i32_const(0);
-    b.i32_const(table.length);
-    b.array_new_data(arrayType, segment);
-    b.global_set(global);
-
-    return global;
-  }
-
   bool _isTypeConstant(DartType type) {
     return type is DynamicType ||
         type is VoidType ||
@@ -727,44 +652,8 @@ class Types {
     } else {
       final ranges =
           translator.classIdNumbering.getConcreteClassIdRanges(interfaceClass);
-      if (ranges.isEmpty) {
-        b.drop();
-        b.i32_const(0);
-      } else if (ranges.length == 1) {
-        final range = ranges[0];
-
-        b.struct_get(translator.topInfo.struct, FieldIndex.classId);
-        b.i32_const(range.start);
-        if (range.length == 1) {
-          b.i32_eq();
-        } else {
-          b.i32_sub();
-          b.i32_const(range.length);
-          b.i32_lt_u();
-        }
-      } else {
-        w.Local idLocal = codeGen.addLocal(w.NumType.i32);
-        b.struct_get(translator.topInfo.struct, FieldIndex.classId);
-        b.local_set(idLocal);
-        w.Label done = b.block(const [], const [w.NumType.i32]);
-        b.i32_const(1);
-
-        for (Range range in ranges) {
-          b.local_get(idLocal);
-          b.i32_const(range.start);
-          if (range.length == 1) {
-            b.i32_eq();
-          } else {
-            b.i32_sub();
-            b.i32_const(range.length);
-            b.i32_lt_u();
-          }
-          b.br_if(done);
-        }
-        b.drop();
-        b.i32_const(0);
-        b.end(); // done
-      }
+      b.struct_get(translator.topInfo.struct, FieldIndex.classId);
+      b.emitClassIdRangeCheck(codeGen, ranges);
     }
 
     if (isPotentiallyNullable) {
diff --git a/sdk/lib/_internal/wasm/lib/class_id.dart b/sdk/lib/_internal/wasm/lib/class_id.dart
index 8be07f90057..45822e12b31 100644
--- a/sdk/lib/_internal/wasm/lib/class_id.dart
+++ b/sdk/lib/_internal/wasm/lib/class_id.dart
@@ -26,14 +26,14 @@ class ClassID {
   external static int get cid_Int8List;
   @pragma("wasm:class-id", "dart.typed_data#_Int8ArrayView")
   external static int get cidInt8ArrayView;
-  @pragma("wasm:class-id", "dart.core#Object")
-  external static int get cidObject;
   @pragma("wasm:class-id", "dart.async#Future")
   external static int get cidFuture;
   @pragma("wasm:class-id", "dart.core#Function")
   external static int get cidFunction;
   @pragma("wasm:class-id", "dart.core#_Closure")
   external static int get cid_Closure;
+  @pragma("wasm:class-id", "dart.core#List")
+  external static int get cidList;
   @pragma("wasm:class-id", "dart.core#_List")
   external static int get cidFixedLengthList;
   @pragma("wasm:class-id", "dart.core#_ListBase")
@@ -69,6 +69,10 @@ class ClassID {
   @pragma("wasm:class-id", "dart.core#_NamedParameter")
   external static int get cidNamedParameter;
 
+  // From this class id onwards, all concrete classes are interface classes and
+  // do not need to be masqueraded.
+  external static int get firstNonMasqueradedInterfaceClassCid;
+
   // Dummy, only used by VM-specific hash table code.
   static final int numPredefinedCids = 1;
 }
diff --git a/sdk/lib/_internal/wasm/lib/core_patch.dart b/sdk/lib/_internal/wasm/lib/core_patch.dart
index 1145a383bf2..511f2ba99a7 100644
--- a/sdk/lib/_internal/wasm/lib/core_patch.dart
+++ b/sdk/lib/_internal/wasm/lib/core_patch.dart
@@ -21,7 +21,9 @@ import "dart:_internal"
         makeFixedListUnmodifiable,
         makeListFixedLength,
         patch,
-        unsafeCast;
+        unsafeCast,
+        WasmStringBase,
+        WasmTypedDataBase;
 
 import "dart:_internal" as _internal show Symbol;
 
@@ -45,7 +47,7 @@ import 'dart:convert' show Encoding, utf8;
 
 import 'dart:math' show Random;
 
-import "dart:typed_data" show Uint8List, Uint16List;
+import "dart:typed_data";
 
 import 'dart:_object_helper';
 import 'dart:_string_helper';
diff --git a/sdk/lib/_internal/wasm/lib/internal_patch.dart b/sdk/lib/_internal/wasm/lib/internal_patch.dart
index 3ea31d8f376..42dec985293 100644
--- a/sdk/lib/_internal/wasm/lib/internal_patch.dart
+++ b/sdk/lib/_internal/wasm/lib/internal_patch.dart
@@ -36,6 +36,12 @@ class Lists {
   }
 }
 
+// Base class for any wasm-backed typed data implementation class.
+abstract class WasmTypedDataBase {}
+
+// Base class for any wasm-backed string implementation class.
+abstract class WasmStringBase implements String {}
+
 // This function can be used to skip implicit or explicit checked down casts in
 // the parts of the core library implementation where we know by construction
 // the type of a value.
diff --git a/sdk/lib/_internal/wasm/lib/simd.dart b/sdk/lib/_internal/wasm/lib/simd.dart
index 27902b54261..d8d05692c77 100644
--- a/sdk/lib/_internal/wasm/lib/simd.dart
+++ b/sdk/lib/_internal/wasm/lib/simd.dart
@@ -10,8 +10,9 @@ import 'dart:_internal'
 import 'dart:collection' show ListMixin;
 import 'dart:math' as math;
 import 'dart:typed_data';
+import 'dart:_internal' show WasmTypedDataBase;
 
-final class NaiveInt32x4List
+final class NaiveInt32x4List extends WasmTypedDataBase
     with ListMixin<Int32x4>, FixedLengthListMixin<Int32x4>
     implements Int32x4List {
   final Int32List _storage;
@@ -116,7 +117,7 @@ final class NaiveUnmodifiableInt32x4List extends NaiveInt32x4List
   ByteBuffer get buffer => UnmodifiableByteBufferView(super.buffer);
 }
 
-final class NaiveFloat32x4List
+final class NaiveFloat32x4List extends WasmTypedDataBase
     with ListMixin<Float32x4>, FixedLengthListMixin<Float32x4>
     implements Float32x4List {
   final Float32List _storage;
@@ -222,7 +223,7 @@ final class NaiveUnmodifiableFloat32x4List extends NaiveFloat32x4List
   ByteBuffer get buffer => UnmodifiableByteBufferView(super.buffer);
 }
 
-final class NaiveFloat64x2List
+final class NaiveFloat64x2List extends WasmTypedDataBase
     with ListMixin<Float64x2>, FixedLengthListMixin<Float64x2>
     implements Float64x2List {
   final Float64List _storage;
@@ -322,7 +323,7 @@ final class NaiveUnmodifiableFloat64x2List extends NaiveFloat64x2List
   ByteBuffer get buffer => UnmodifiableByteBufferView(super.buffer);
 }
 
-final class NaiveFloat32x4 implements Float32x4 {
+final class NaiveFloat32x4 extends WasmTypedDataBase implements Float32x4 {
   final double x;
   final double y;
   final double z;
@@ -614,7 +615,7 @@ final class NaiveFloat32x4 implements Float32x4 {
   }
 }
 
-final class NaiveFloat64x2 implements Float64x2 {
+final class NaiveFloat64x2 extends WasmTypedDataBase implements Float64x2 {
   final double x;
   final double y;
 
@@ -688,7 +689,7 @@ final class NaiveFloat64x2 implements Float64x2 {
   Float64x2 sqrt() => NaiveFloat64x2._doubles(math.sqrt(x), math.sqrt(y));
 }
 
-final class NaiveInt32x4 implements Int32x4 {
+final class NaiveInt32x4 extends WasmTypedDataBase implements Int32x4 {
   final int x;
   final int y;
   final int z;
diff --git a/sdk/lib/_internal/wasm/lib/string.dart b/sdk/lib/_internal/wasm/lib/string.dart
index e45710d1c8c..3989b37e043 100644
--- a/sdk/lib/_internal/wasm/lib/string.dart
+++ b/sdk/lib/_internal/wasm/lib/string.dart
@@ -10,7 +10,8 @@ import "dart:_internal"
         ClassID,
         EfficientLengthIterable,
         makeListFixedLength,
-        unsafeCast;
+        unsafeCast,
+        WasmStringBase;
 
 import 'dart:_js_helper' show JS;
 import 'dart:_js_types' show JSStringImpl;
@@ -70,7 +71,7 @@ String _toLowerCase(String string) => JS<String>(
  * [StringBase] contains common methods used by concrete String
  * implementations, e.g., OneByteString.
  */
-abstract final class StringBase implements String {
+abstract final class StringBase extends WasmStringBase {
   bool _isWhitespace(int codeUnit);
 
   // Constants used by replaceAll encoding of string slices between matches.
diff --git a/sdk/lib/_internal/wasm/lib/type.dart b/sdk/lib/_internal/wasm/lib/type.dart
index 4827242910e..4ba192e184c 100644
--- a/sdk/lib/_internal/wasm/lib/type.dart
+++ b/sdk/lib/_internal/wasm/lib/type.dart
@@ -515,7 +515,7 @@ class _AbstractRecordType extends _Type {
 
   @override
   bool _checkInstance(Object o) {
-    return _isRecordInstance(o);
+    return _isRecordClassId(ClassID.getID(o));
   }
 
   @override
@@ -537,7 +537,7 @@ class _RecordType extends _Type {
 
   @override
   bool _checkInstance(Object o) {
-    if (!_isRecordInstance(o)) return false;
+    if (!_isRecordClassId(ClassID.getID(o))) return false;
     return unsafeCast<Record>(o)._checkRecordType(fieldTypes, names);
   }
 
@@ -605,13 +605,6 @@ external WasmArray<WasmArray<WasmI32>> _getTypeRulesSupers();
 external WasmArray<WasmArray<WasmArray<_Type>>> _getTypeRulesSubstitutions();
 external WasmArray<String>? _getTypeNames();
 
-external WasmArray<WasmI8> get _typeCategoryTable;
-external WasmI32 get _typeCategoryAbstractClass;
-external WasmI32 get _typeCategoryObject;
-external WasmI32 get _typeCategoryFunction;
-external WasmI32 get _typeCategoryRecord;
-external WasmI32 get _typeCategoryNotMasqueraded;
-
 /// Type parameter environment used while comparing function types.
 ///
 /// In the case of nested function types, the environment refers to the
@@ -1299,20 +1292,14 @@ void _checkClosureType(
 
 _Type _getActualRuntimeType(Object object) {
   final classId = ClassID.getID(object);
-  final category = _typeCategoryTable.readUnsigned(classId);
 
-  if (category == _typeCategoryFunction.toIntUnsigned()) {
+  if (_isObjectClassId(classId)) return _literal<Object>();
+  if (_isRecordClassId(classId)) {
+    return Record._getMasqueradedRecordRuntimeType(unsafeCast<Record>(object));
+  }
+  if (_isClosureClassId(classId)) {
     return _Closure._getClosureRuntimeType(unsafeCast<_Closure>(object));
   }
-  if (category == _typeCategoryRecord.toIntUnsigned()) {
-    return Record._getRecordRuntimeType(unsafeCast<Record>(object));
-  }
-  if (category == _typeCategoryObject.toIntUnsigned()) {
-    return _literal<Object>();
-  }
-  if (category == _typeCategoryAbstractClass.toIntUnsigned()) {
-    throw 'unreachable';
-  }
   return _InterfaceType(classId, false, Object._getTypeArguments(object));
 }
 
@@ -1323,28 +1310,98 @@ _Type _getActualRuntimeTypeNullable(Object? object) =>
 @pragma("wasm:entry-point")
 _Type _getMasqueradedRuntimeType(Object object) {
   final classId = ClassID.getID(object);
-  final category = _typeCategoryTable.readUnsigned(classId);
 
-  if (category == _typeCategoryNotMasqueraded.toIntUnsigned()) {
+  // Fast path: Most usages of `.runtimeType` may be on user-defined classes
+  // (e.g. `Widget.runtimeType`, ...)
+  if (ClassID.firstNonMasqueradedInterfaceClassCid <= classId) {
+    // Non-masqueraded interface type.
     return _InterfaceType(classId, false, Object._getTypeArguments(object));
   }
-  if (category == _typeCategoryFunction.toIntUnsigned()) {
-    return _Closure._getClosureRuntimeType(unsafeCast<_Closure>(object));
-  }
-  if (category == _typeCategoryRecord.toIntUnsigned()) {
+
+  if (_isObjectClassId(classId)) return _literal<Object>();
+  if (_isRecordClassId(classId)) {
     return Record._getMasqueradedRecordRuntimeType(unsafeCast<Record>(object));
   }
-  if (category == _typeCategoryObject.toIntUnsigned()) {
-    return _literal<Object>();
+  if (_isClosureClassId(classId)) {
+    return _Closure._getClosureRuntimeType(unsafeCast<_Closure>(object));
   }
-  if (category == _typeCategoryAbstractClass.toIntUnsigned()) {
-    throw 'unreachable';
+
+  // The object is a normal instance of a class (not function or record).
+
+  const bool isJsCompatibility =
+      bool.fromEnvironment('dart.wasm.js_compatibility');
+
+  // This method is not used in the RTT implementation, it's purely used for
+  // producing `Type` objects for `<obj>.runtimeType`.
+  //
+  // => We can use normal `is` checks in here that will be desugared to class-id
+  //    range checks.
+
+  if (object is bool) return _literal<bool>();
+  if (object is int) return _literal<int>();
+  if (object is double) return _literal<double>();
+  if (object is _Type) return _literal<Type>();
+  if (object is _ListBase) {
+    return _InterfaceType(
+        ClassID.cidList, false, Object._getTypeArguments(object));
   }
-  return _InterfaceType(category, false, Object._getTypeArguments(object));
+
+  if (isJsCompatibility) {
+    if (object is String) return _literal<String>();
+    if (object is TypedData) {
+      if (object is ByteData) return _literal<ByteData>();
+      if (object is Int8List) return _literal<Int8List>();
+      if (object is Uint8List) return _literal<Uint8List>();
+      if (object is Uint8ClampedList) return _literal<Uint8ClampedList>();
+      if (object is Int16List) return _literal<Int16List>();
+      if (object is Uint16List) return _literal<Uint16List>();
+      if (object is Int32List) return _literal<Int32List>();
+      if (object is Uint32List) return _literal<Uint32List>();
+      if (object is Int64List) return _literal<Int64List>();
+      if (object is Uint64List) return _literal<Uint64List>();
+      if (object is Float32List) return _literal<Float32List>();
+      if (object is Float64List) return _literal<Float64List>();
+      if (object is Int32x4List) return _literal<Int32x4List>();
+      if (object is Float32x4List) return _literal<Float32x4List>();
+      if (object is Float64x2List) return _literal<Float64x2List>();
+    }
+    if (object is ByteBuffer) return _literal<ByteBuffer>();
+    if (object is Float32x4) return _literal<Float32x4>();
+    if (object is Float64x2) return _literal<Float64x2>();
+    if (object is Int32x4) return _literal<Int32x4>();
+  } else {
+    if (object is WasmStringBase) return _literal<String>();
+    if (object is WasmTypedDataBase) {
+      if (object is ByteData) return _literal<ByteData>();
+      if (object is Int8List) return _literal<Int8List>();
+      if (object is Uint8List) return _literal<Uint8List>();
+      if (object is Uint8ClampedList) return _literal<Uint8ClampedList>();
+      if (object is Int16List) return _literal<Int16List>();
+      if (object is Uint16List) return _literal<Uint16List>();
+      if (object is Int32List) return _literal<Int32List>();
+      if (object is Uint32List) return _literal<Uint32List>();
+      if (object is Int64List) return _literal<Int64List>();
+      if (object is Uint64List) return _literal<Uint64List>();
+      if (object is Float32List) return _literal<Float32List>();
+      if (object is Float64List) return _literal<Float64List>();
+      if (object is Int32x4List) return _literal<Int32x4List>();
+      if (object is Float32x4List) return _literal<Float32x4List>();
+      if (object is Float64x2List) return _literal<Float64x2List>();
+      if (object is ByteBuffer) return _literal<ByteBuffer>();
+      if (object is Float32x4) return _literal<Float32x4>();
+      if (object is Float64x2) return _literal<Float64x2>();
+      if (object is Int32x4) return _literal<Int32x4>();
+    }
+  }
+
+  // Non-masqueraded interface type.
+  return _InterfaceType(classId, false, Object._getTypeArguments(object));
 }
 
 @pragma("wasm:prefer-inline")
 _Type _getMasqueradedRuntimeTypeNullable(Object? object) =>
     object == null ? _literal<Null>() : _getMasqueradedRuntimeType(object);
 
-external bool _isRecordInstance(Object o);
+external bool _isObjectClassId(int classId);
+external bool _isClosureClassId(int classId);
+external bool _isRecordClassId(int classId);
diff --git a/sdk/lib/_internal/wasm/lib/typed_data.dart b/sdk/lib/_internal/wasm/lib/typed_data.dart
index 7fdfdb41eab..5a0e45d00f5 100644
--- a/sdk/lib/_internal/wasm/lib/typed_data.dart
+++ b/sdk/lib/_internal/wasm/lib/typed_data.dart
@@ -22,6 +22,7 @@ import 'dart:_internal'
         SubListIterable,
         TakeWhileIterable,
         unsafeCast,
+        WasmTypedDataBase,
         WhereIterable,
         WhereTypeIterable;
 import 'dart:_simd';
@@ -101,7 +102,7 @@ final class _TypedListIterator<E> implements Iterator<E> {
 /// and [_setUint8Unchecked] methods. Implementations should implement these
 /// methods and override get/set methods for elements matching the buffer
 /// element type to provide fast access.
-abstract class ByteDataBase implements ByteData {
+abstract class ByteDataBase extends WasmTypedDataBase implements ByteData {
   final int offsetInBytes;
   final int lengthInBytes;
 
@@ -964,7 +965,7 @@ class _UnmodifiableF64ByteData extends _F64ByteData
 /// Base class for [ByteBuffer] implementations. Returns slow lists in all
 /// methods. Implementations should override relevant methods to return fast
 /// lists when possible and implement [asByteData].
-abstract class ByteBufferBase extends ByteBuffer {
+abstract class ByteBufferBase extends WasmTypedDataBase implements ByteBuffer {
   final int lengthInBytes;
   final bool _mutable;
 
@@ -1332,7 +1333,8 @@ class _F64ByteBuffer extends ByteBufferBase {
   }
 }
 
-class UnmodifiableByteBuffer implements UnmodifiableByteBufferView {
+class UnmodifiableByteBuffer extends WasmTypedDataBase
+    implements UnmodifiableByteBufferView {
   final ByteBufferBase _buffer;
 
   UnmodifiableByteBuffer(ByteBufferBase buffer) : _buffer = buffer._immutable();
@@ -2242,7 +2244,7 @@ mixin _UnmodifiableDoubleListMixin {
 // Fast lists
 //
 
-abstract class _WasmI8ArrayBase {
+abstract class _WasmI8ArrayBase extends WasmTypedDataBase {
   final WasmArray<WasmI8> _data;
   final int _offsetInElements;
   final int length;
@@ -2260,7 +2262,7 @@ abstract class _WasmI8ArrayBase {
   ByteBuffer get buffer => _I8ByteBuffer(_data);
 }
 
-abstract class _WasmI16ArrayBase {
+abstract class _WasmI16ArrayBase extends WasmTypedDataBase {
   final WasmArray<WasmI16> _data;
   final int _offsetInElements;
   final int length;
@@ -2278,7 +2280,7 @@ abstract class _WasmI16ArrayBase {
   ByteBuffer get buffer => _I16ByteBuffer(_data);
 }
 
-abstract class _WasmI32ArrayBase {
+abstract class _WasmI32ArrayBase extends WasmTypedDataBase {
   final WasmArray<WasmI32> _data;
   final int _offsetInElements;
   final int length;
@@ -2296,7 +2298,7 @@ abstract class _WasmI32ArrayBase {
   ByteBuffer get buffer => _I32ByteBuffer(_data);
 }
 
-abstract class _WasmI64ArrayBase {
+abstract class _WasmI64ArrayBase extends WasmTypedDataBase {
   final WasmArray<WasmI64> _data;
   final int _offsetInElements;
   final int length;
@@ -2314,7 +2316,7 @@ abstract class _WasmI64ArrayBase {
   ByteBuffer get buffer => _I64ByteBuffer(_data);
 }
 
-abstract class _WasmF32ArrayBase {
+abstract class _WasmF32ArrayBase extends WasmTypedDataBase {
   final WasmArray<WasmF32> _data;
   final int _offsetInElements;
   final int length;
@@ -2332,7 +2334,7 @@ abstract class _WasmF32ArrayBase {
   ByteBuffer get buffer => _F32ByteBuffer(_data);
 }
 
-abstract class _WasmF64ArrayBase {
+abstract class _WasmF64ArrayBase extends WasmTypedDataBase {
   final WasmArray<WasmF64> _data;
   final int _offsetInElements;
   final int length;
@@ -2931,7 +2933,7 @@ class UnmodifiableF64List extends F64List
 // Slow lists
 //
 
-class _SlowListBase {
+class _SlowListBase extends WasmTypedDataBase {
   final ByteBuffer buffer;
   final int offsetInBytes;
   final int length;
diff --git a/tests/web/wasm/masquerade_test.dart b/tests/web/wasm/masquerade_test.dart
new file mode 100644
index 00000000000..43f222991d6
--- /dev/null
+++ b/tests/web/wasm/masquerade_test.dart
@@ -0,0 +1,35 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:typed_data';
+import 'dart:js_interop';
+
+import 'package:expect/expect.dart';
+
+const bool isJsCompatibility =
+    bool.fromEnvironment('dart.wasm.js_compatibility');
+
+void main() {
+  final uint8List = Uint8List(10);
+  final uint8ListView = uint8List.buffer.asUint8List(1, 2);
+  final uint16ListView = uint8List.buffer.asUint16List(2, 4);
+
+  Expect.equals('Uint8List', uint8List.runtimeType.toString());
+  Expect.equals('Uint8List', uint8ListView.runtimeType.toString());
+  Expect.equals('Uint16List', uint16ListView.runtimeType.toString());
+
+  final jsString = eval('"foobar"').dartify();
+  final jsTypedData = eval('new Uint8Array(10)').dartify();
+
+  if (isJsCompatibility) {
+    Expect.equals('String', jsString.runtimeType.toString());
+    Expect.equals('Uint8List', jsTypedData.runtimeType.toString());
+  } else {
+    Expect.equals('JSStringImpl', jsString.runtimeType.toString());
+    Expect.equals('JSUint8ArrayImpl', jsTypedData.runtimeType.toString());
+  }
+}
+
+@JS()
+external JSObject eval(String code);
diff --git a/utils/dart2wasm/BUILD.gn b/utils/dart2wasm/BUILD.gn
index ca7d50784bf..455e2622327 100644
--- a/utils/dart2wasm/BUILD.gn
+++ b/utils/dart2wasm/BUILD.gn
@@ -53,7 +53,6 @@ compile_platform("compile_dart2wasm_platform") {
 
   args = [
     "--target=dart2wasm",
-    "--no-defines",
     "dart:core",
     "--nnbd-strong",
   ]
@@ -71,7 +70,7 @@ compile_platform("compile_dart2wasm_js_compatibility_platform") {
 
   args = [
     "--target=dart2wasm_js_compatibility",
-    "--no-defines",
+    "-Ddart.wasm.js_compatibility=true",
     "dart:core",
     "--nnbd-strong",
   ]