diff --git a/sdk/lib/js_util/js_util.dart b/sdk/lib/js_util/js_util.dart
index fcd668cbe0a..5dca72d6c7b 100644
--- a/sdk/lib/js_util/js_util.dart
+++ b/sdk/lib/js_util/js_util.dart
@@ -68,7 +68,8 @@ T newObject<T>() => JS('=Object', '{}');
 
 bool hasProperty(Object o, Object name) => JS('bool', '# in #', name, o);
 
-T getProperty<T>(Object o, Object name) => JS('Object|Null', '#[#]', o, name);
+T getProperty<T>(Object o, Object name) =>
+    JS<dynamic>('Object|Null', '#[#]', o, name);
 
 // A CFE transformation may optimize calls to `setProperty`, when [value] is
 // statically known to be a non-function.
@@ -88,40 +89,41 @@ T _setPropertyUnchecked<T>(Object o, Object name, T? value) {
 // statically known to be non-functions.
 T callMethod<T>(Object o, String method, List<Object?> args) {
   assertInteropArgs(args);
-  return JS('Object|Null', '#[#].apply(#, #)', o, method, o, args);
+  return JS<dynamic>('Object|Null', '#[#].apply(#, #)', o, method, o, args);
 }
 
 /// Unchecked version for 0 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callMethodUnchecked0<T>(Object o, String method) {
-  return JS('Object|Null', '#[#]()', o, method);
+  return JS<dynamic>('Object|Null', '#[#]()', o, method);
 }
 
 /// Unchecked version for 1 argument, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callMethodUnchecked1<T>(Object o, String method, Object? arg1) {
-  return JS('Object|Null', '#[#](#)', o, method, arg1);
+  return JS<dynamic>('Object|Null', '#[#](#)', o, method, arg1);
 }
 
 /// Unchecked version for 2 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callMethodUnchecked2<T>(
     Object o, String method, Object? arg1, Object? arg2) {
-  return JS('Object|Null', '#[#](#, #)', o, method, arg1, arg2);
+  return JS<dynamic>('Object|Null', '#[#](#, #)', o, method, arg1, arg2);
 }
 
 /// Unchecked version for 3 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callMethodUnchecked3<T>(
     Object o, String method, Object? arg1, Object? arg2, Object? arg3) {
-  return JS('Object|Null', '#[#](#, #, #)', o, method, arg1, arg2, arg3);
+  return JS<dynamic>(
+      'Object|Null', '#[#](#, #, #)', o, method, arg1, arg2, arg3);
 }
 
 /// Unchecked version for 4 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callMethodUnchecked4<T>(Object o, String method, Object? arg1, Object? arg2,
     Object? arg3, Object? arg4) {
-  return JS(
+  return JS<dynamic>(
       'Object|Null', '#[#](#, #, #, #)', o, method, arg1, arg2, arg3, arg4);
 }
 
@@ -134,7 +136,7 @@ bool instanceof(Object? o, Object type) =>
 
 T callConstructor<T>(Object constr, List<Object?>? arguments) {
   if (arguments == null) {
-    return JS('Object', 'new #()', constr);
+    return JS<dynamic>('Object', 'new #()', constr);
   } else {
     assertInteropArgs(arguments);
   }
@@ -143,29 +145,30 @@ T callConstructor<T>(Object constr, List<Object?>? arguments) {
     int argumentCount = JS('int', '#.length', arguments);
     switch (argumentCount) {
       case 0:
-        return JS('Object', 'new #()', constr);
+        return JS<dynamic>('Object', 'new #()', constr);
 
       case 1:
         var arg0 = JS('', '#[0]', arguments);
-        return JS('Object', 'new #(#)', constr, arg0);
+        return JS<dynamic>('Object', 'new #(#)', constr, arg0);
 
       case 2:
         var arg0 = JS('', '#[0]', arguments);
         var arg1 = JS('', '#[1]', arguments);
-        return JS('Object', 'new #(#, #)', constr, arg0, arg1);
+        return JS<dynamic>('Object', 'new #(#, #)', constr, arg0, arg1);
 
       case 3:
         var arg0 = JS('', '#[0]', arguments);
         var arg1 = JS('', '#[1]', arguments);
         var arg2 = JS('', '#[2]', arguments);
-        return JS('Object', 'new #(#, #, #)', constr, arg0, arg1, arg2);
+        return JS<dynamic>(
+            'Object', 'new #(#, #, #)', constr, arg0, arg1, arg2);
 
       case 4:
         var arg0 = JS('', '#[0]', arguments);
         var arg1 = JS('', '#[1]', arguments);
         var arg2 = JS('', '#[2]', arguments);
         var arg3 = JS('', '#[3]', arguments);
-        return JS(
+        return JS<dynamic>(
             'Object', 'new #(#, #, #, #)', constr, arg0, arg1, arg2, arg3);
     }
   }
@@ -183,7 +186,7 @@ T callConstructor<T>(Object constr, List<Object?>? arguments) {
   JS('String', 'String(#)', factoryFunction);
   // This could return an UnknownJavaScriptObject, or a native
   // object for which there is an interceptor
-  return JS('Object', 'new #()', factoryFunction);
+  return JS<dynamic>('Object', 'new #()', factoryFunction);
 
   // TODO(sra): Investigate:
   //
@@ -196,33 +199,34 @@ T callConstructor<T>(Object constr, List<Object?>? arguments) {
 /// Unchecked version for 0 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callConstructorUnchecked0<T>(Object constr) {
-  return JS('Object', 'new #()', constr);
+  return JS<dynamic>('Object', 'new #()', constr);
 }
 
 /// Unchecked version for 1 argument, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callConstructorUnchecked1<T>(Object constr, Object? arg1) {
-  return JS('Object', 'new #(#)', constr, arg1);
+  return JS<dynamic>('Object', 'new #(#)', constr, arg1);
 }
 
 /// Unchecked version for 2 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callConstructorUnchecked2<T>(Object constr, Object? arg1, Object? arg2) {
-  return JS('Object', 'new #(#, #)', constr, arg1, arg2);
+  return JS<dynamic>('Object', 'new #(#, #)', constr, arg1, arg2);
 }
 
 /// Unchecked version for 3 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callConstructorUnchecked3<T>(
     Object constr, Object? arg1, Object? arg2, Object? arg3) {
-  return JS('Object', 'new #(#, #, #)', constr, arg1, arg2, arg3);
+  return JS<dynamic>('Object', 'new #(#, #, #)', constr, arg1, arg2, arg3);
 }
 
 /// Unchecked version for 4 arguments, only used in a CFE transformation.
 @pragma('dart2js:tryInline')
 T _callConstructorUnchecked4<T>(
     Object constr, Object? arg1, Object? arg2, Object? arg3, Object? arg4) {
-  return JS('Object', 'new #(#, #, #, #)', constr, arg1, arg2, arg3, arg4);
+  return JS<dynamic>(
+      'Object', 'new #(#, #, #, #)', constr, arg1, arg2, arg3, arg4);
 }
 
 /// Exception for when the promise is rejected with a `null` or `undefined`
diff --git a/tests/lib/js/js_util/implicit_downcast_test.dart b/tests/lib/js/js_util/implicit_downcast_test.dart
new file mode 100644
index 00000000000..b0d201b1bd3
--- /dev/null
+++ b/tests/lib/js/js_util/implicit_downcast_test.dart
@@ -0,0 +1,124 @@
+// Copyright (c) 2022, 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.
+
+// Tests implicit downcasts in js_util.
+
+@JS()
+library js_util_implicit_downcast_test;
+
+import 'package:js/js.dart';
+import 'package:js/js_util.dart' as js_util;
+import 'package:expect/minitest.dart';
+
+@JS()
+external void eval(String code);
+
+@JS()
+class Foo {
+  external Foo(num a);
+
+  external num get a;
+  external void set a(_);
+  external num bar();
+}
+
+bool isComplianceMode() {
+  var stuff = [1, 'string'];
+  var a = stuff[0];
+  // Detect whether we are using --omit-implicit-checks.
+  try {
+    String s = a as dynamic;
+    return false;
+  } catch (e) {
+    // Ignore.
+  }
+  return true;
+}
+
+main() {
+  eval(r"""
+    function Foo(a) {
+      this.a = a;
+    }
+
+    Foo.prototype.bar = function() {
+      return this.a;
+    }
+    """);
+
+  if (isComplianceMode()) {
+    complianceModeTest();
+  } else {
+    omitImplicitChecksTest();
+  }
+}
+
+complianceModeTest() {
+  var f = Foo(42);
+  expect(js_util.getProperty<int>(f, 'a'), equals(42));
+  expect(() => js_util.getProperty<List>(f, 'a'), throws);
+
+  f.a = 5;
+  expect(js_util.callMethod<int>(f, 'bar', []), equals(5));
+  expect(() => js_util.callMethod<List>(f, 'bar', []), throws);
+
+  // Check optimized lowering of callMethod.
+  expect(() => js_util.callMethod<List>(f, 'bar', [1]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2, 3]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4, 5]), throws);
+
+  var f2 = Foo(7);
+  var fConstructor = js_util.getProperty(f, 'constructor');
+  expect(js_util.callConstructor<Foo>(fConstructor, [7]).a, equals(7));
+  expect(() => js_util.callConstructor<List>(fConstructor, [7]), throws);
+
+  // Check optimized lowering of callConstructor.
+  expect(() => js_util.callConstructor<List>(fConstructor, null), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, []), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, [1, 2]), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, [1, 2, 3]), throws);
+  expect(
+      () => js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4]), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4, 5]),
+      throws);
+}
+
+omitImplicitChecksTest() {
+  var f = Foo(42);
+  expect(js_util.getProperty<int>(f, 'a'), equals(42));
+  expect(js_util.getProperty<List>(f, 'a'), equals(42));
+
+  f.a = 5;
+  expect(js_util.callMethod<int>(f, 'bar', []), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', []), equals(5));
+
+  // Check optimized lowering of callMethod.
+  expect(js_util.callMethod<List>(f, 'bar', [1]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2, 3]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4, 5]), equals(5));
+
+  var fConstructor = js_util.getProperty(f, 'constructor');
+  expect(js_util.callConstructor<Foo>(fConstructor, [7]).a, equals(7));
+  expect(
+      (js_util.callConstructor<List>(fConstructor, [7]) as Foo).a, equals(7));
+
+  // Check optimized lowering of callConstructor.
+  expect((js_util.callConstructor<List>(fConstructor, null) as Foo).a,
+      equals(null));
+  expect(
+      (js_util.callConstructor<List>(fConstructor, []) as Foo).a, equals(null));
+  expect((js_util.callConstructor<List>(fConstructor, [1, 2]) as Foo).a,
+      equals(1));
+  expect((js_util.callConstructor<List>(fConstructor, [1, 2, 3]) as Foo).a,
+      equals(1));
+  expect((js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4]) as Foo).a,
+      equals(1));
+  expect(
+      (js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4, 5]) as Foo).a,
+      equals(1));
+}
diff --git a/tests/lib/lib.status b/tests/lib/lib.status
index 46bdedf6860..770c4d6f56a 100644
--- a/tests/lib/lib.status
+++ b/tests/lib/lib.status
@@ -45,6 +45,7 @@ js/external_extension_members_test: SkipByDesign # Issue 42085. CSP policy disal
 js/instanceof_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/is_check_and_as_cast_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/async_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
+js/js_util/implicit_downcast_test.dart: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/javascriptobject_extensions_test.dart: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/jsify_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/promise_reject_null_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
diff --git a/tests/lib_2/js/js_util/implicit_downcast_test.dart b/tests/lib_2/js/js_util/implicit_downcast_test.dart
new file mode 100644
index 00000000000..b0d201b1bd3
--- /dev/null
+++ b/tests/lib_2/js/js_util/implicit_downcast_test.dart
@@ -0,0 +1,124 @@
+// Copyright (c) 2022, 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.
+
+// Tests implicit downcasts in js_util.
+
+@JS()
+library js_util_implicit_downcast_test;
+
+import 'package:js/js.dart';
+import 'package:js/js_util.dart' as js_util;
+import 'package:expect/minitest.dart';
+
+@JS()
+external void eval(String code);
+
+@JS()
+class Foo {
+  external Foo(num a);
+
+  external num get a;
+  external void set a(_);
+  external num bar();
+}
+
+bool isComplianceMode() {
+  var stuff = [1, 'string'];
+  var a = stuff[0];
+  // Detect whether we are using --omit-implicit-checks.
+  try {
+    String s = a as dynamic;
+    return false;
+  } catch (e) {
+    // Ignore.
+  }
+  return true;
+}
+
+main() {
+  eval(r"""
+    function Foo(a) {
+      this.a = a;
+    }
+
+    Foo.prototype.bar = function() {
+      return this.a;
+    }
+    """);
+
+  if (isComplianceMode()) {
+    complianceModeTest();
+  } else {
+    omitImplicitChecksTest();
+  }
+}
+
+complianceModeTest() {
+  var f = Foo(42);
+  expect(js_util.getProperty<int>(f, 'a'), equals(42));
+  expect(() => js_util.getProperty<List>(f, 'a'), throws);
+
+  f.a = 5;
+  expect(js_util.callMethod<int>(f, 'bar', []), equals(5));
+  expect(() => js_util.callMethod<List>(f, 'bar', []), throws);
+
+  // Check optimized lowering of callMethod.
+  expect(() => js_util.callMethod<List>(f, 'bar', [1]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2, 3]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4]), throws);
+  expect(() => js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4, 5]), throws);
+
+  var f2 = Foo(7);
+  var fConstructor = js_util.getProperty(f, 'constructor');
+  expect(js_util.callConstructor<Foo>(fConstructor, [7]).a, equals(7));
+  expect(() => js_util.callConstructor<List>(fConstructor, [7]), throws);
+
+  // Check optimized lowering of callConstructor.
+  expect(() => js_util.callConstructor<List>(fConstructor, null), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, []), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, [1, 2]), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, [1, 2, 3]), throws);
+  expect(
+      () => js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4]), throws);
+  expect(() => js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4, 5]),
+      throws);
+}
+
+omitImplicitChecksTest() {
+  var f = Foo(42);
+  expect(js_util.getProperty<int>(f, 'a'), equals(42));
+  expect(js_util.getProperty<List>(f, 'a'), equals(42));
+
+  f.a = 5;
+  expect(js_util.callMethod<int>(f, 'bar', []), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', []), equals(5));
+
+  // Check optimized lowering of callMethod.
+  expect(js_util.callMethod<List>(f, 'bar', [1]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2, 3]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4]), equals(5));
+  expect(js_util.callMethod<List>(f, 'bar', [1, 2, 3, 4, 5]), equals(5));
+
+  var fConstructor = js_util.getProperty(f, 'constructor');
+  expect(js_util.callConstructor<Foo>(fConstructor, [7]).a, equals(7));
+  expect(
+      (js_util.callConstructor<List>(fConstructor, [7]) as Foo).a, equals(7));
+
+  // Check optimized lowering of callConstructor.
+  expect((js_util.callConstructor<List>(fConstructor, null) as Foo).a,
+      equals(null));
+  expect(
+      (js_util.callConstructor<List>(fConstructor, []) as Foo).a, equals(null));
+  expect((js_util.callConstructor<List>(fConstructor, [1, 2]) as Foo).a,
+      equals(1));
+  expect((js_util.callConstructor<List>(fConstructor, [1, 2, 3]) as Foo).a,
+      equals(1));
+  expect((js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4]) as Foo).a,
+      equals(1));
+  expect(
+      (js_util.callConstructor<List>(fConstructor, [1, 2, 3, 4, 5]) as Foo).a,
+      equals(1));
+}
diff --git a/tests/lib_2/lib_2.status b/tests/lib_2/lib_2.status
index bef1d690958..d34ce69003d 100644
--- a/tests/lib_2/lib_2.status
+++ b/tests/lib_2/lib_2.status
@@ -45,6 +45,7 @@ js/external_extension_members_test: SkipByDesign # Issue 42085. CSP policy disal
 js/instanceof_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/is_check_and_as_cast_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/async_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
+js/js_util/implicit_downcast_test.dart: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/javascriptobject_extensions_test.dart: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/jsify_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code
 js/js_util/promise_reject_null_test: SkipByDesign # Issue 42085. CSP policy disallows injected JS code