diff --git a/pkg/compiler/lib/src/serialization/binary_sink.dart b/pkg/compiler/lib/src/serialization/binary_sink.dart
index c5c52a7175c..c408ccca80a 100644
--- a/pkg/compiler/lib/src/serialization/binary_sink.dart
+++ b/pkg/compiler/lib/src/serialization/binary_sink.dart
@@ -2,16 +2,17 @@
 // 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.
 
-// @dart = 2.10
-
-part of 'serialization.dart';
+import 'dart:convert';
+import 'package:kernel/binary/ast_to_binary.dart';
+import 'data_sink.dart';
 
 /// [DataSink] that writes data as a sequence of bytes.
 ///
 /// This data sink works together with [BinarySourceWriter].
 class BinaryDataSink implements DataSink {
   final Sink<List<int>> sink;
-  BufferedSink _bufferedSink;
+  // Nullable and non-final to allow storage to be released.
+  BufferedSink? _bufferedSink;
   int _length = 0;
 
   BinaryDataSink(this.sink) : _bufferedSink = BufferedSink(sink);
@@ -33,7 +34,7 @@ class BinaryDataSink implements DataSink {
   void writeString(String value) {
     List<int> bytes = utf8.encode(value);
     writeInt(bytes.length);
-    _bufferedSink.addBytes(bytes);
+    _bufferedSink!.addBytes(bytes);
     _length += bytes.length;
   }
 
@@ -41,13 +42,13 @@ class BinaryDataSink implements DataSink {
   void writeInt(int value) {
     assert(value >= 0 && value >> 30 == 0);
     if (value < 0x80) {
-      _bufferedSink.addByte(value);
+      _bufferedSink!.addByte(value);
       _length += 1;
     } else if (value < 0x4000) {
-      _bufferedSink.addByte2((value >> 8) | 0x80, value & 0xFF);
+      _bufferedSink!.addByte2((value >> 8) | 0x80, value & 0xFF);
       _length += 2;
     } else {
-      _bufferedSink.addByte4((value >> 24) | 0xC0, (value >> 16) & 0xFF,
+      _bufferedSink!.addByte4((value >> 24) | 0xC0, (value >> 16) & 0xFF,
           (value >> 8) & 0xFF, value & 0xFF);
       _length += 4;
     }
@@ -60,7 +61,7 @@ class BinaryDataSink implements DataSink {
 
   @override
   void close() {
-    _bufferedSink.flushAndDestroy();
+    _bufferedSink!.flushAndDestroy();
     _bufferedSink = null;
     sink.close();
   }
diff --git a/pkg/compiler/lib/src/serialization/binary_source.dart b/pkg/compiler/lib/src/serialization/binary_source.dart
index 00d1291d37b..59f40ffe3a7 100644
--- a/pkg/compiler/lib/src/serialization/binary_source.dart
+++ b/pkg/compiler/lib/src/serialization/binary_source.dart
@@ -2,9 +2,10 @@
 // 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.
 
-// @dart = 2.10
-
-part of 'serialization.dart';
+import 'dart:convert';
+import 'dart:typed_data';
+import 'data_source.dart';
+import 'serialization_interfaces.dart' show StringInterner;
 
 /// [DataSource] that reads data from a sequence of bytes.
 ///
@@ -12,10 +13,12 @@ part of 'serialization.dart';
 class BinaryDataSource implements DataSource {
   int _byteOffset = 0;
   final List<int> _bytes;
-  final StringInterner _stringInterner;
+  final StringInterner? _stringInterner;
 
-  BinaryDataSource(this._bytes, {StringInterner stringInterner})
-      : _stringInterner = stringInterner;
+  BinaryDataSource(this._bytes, {StringInterner? stringInterner})
+      : _stringInterner = stringInterner {
+    assert((_bytes as dynamic) != null); // TODO(48820): Remove when sound.
+  }
 
   @override
   void begin(String tag) {}
@@ -33,7 +36,7 @@ class BinaryDataSource implements DataSource {
     _byteOffset += bytes.length;
     String string = utf8.decode(bytes);
     if (_stringInterner == null) return string;
-    return _stringInterner.internString(string);
+    return _stringInterner!.internString(string);
   }
 
   @override
diff --git a/pkg/compiler/lib/src/serialization/data_sink.dart b/pkg/compiler/lib/src/serialization/data_sink.dart
new file mode 100644
index 00000000000..b87ebcd921c
--- /dev/null
+++ b/pkg/compiler/lib/src/serialization/data_sink.dart
@@ -0,0 +1,30 @@
+// Copyright (c) 2018, 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.
+
+/// Interface handling [DataSinkWriter] low-level data serialization.
+///
+/// Each implementation of [DataSink] should have a corresponding
+/// [DataSource] that deserializes data serialized by that implementation.
+// TODO(48820): Move this interface back to 'sink.dart'.
+abstract class DataSink {
+  int get length;
+
+  /// Serialization of a non-negative integer value.
+  void writeInt(int value);
+
+  /// Serialization of an enum value.
+  void writeEnum(dynamic value);
+
+  /// Serialization of a String value.
+  void writeString(String value);
+
+  /// Serialization of a section begin tag. May be omitted by some writers.
+  void beginTag(String tag);
+
+  /// Serialization of a section end tag. May be omitted by some writers.
+  void endTag(String tag);
+
+  /// Closes any underlying data sinks.
+  void close();
+}
diff --git a/pkg/compiler/lib/src/serialization/data_source.dart b/pkg/compiler/lib/src/serialization/data_source.dart
new file mode 100644
index 00000000000..ee73699fd19
--- /dev/null
+++ b/pkg/compiler/lib/src/serialization/data_source.dart
@@ -0,0 +1,29 @@
+// Copyright (c) 2018, 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.
+
+/// Interface handling [DataSourceReader] low-level data deserialization.
+///
+/// Each implementation of [DataSource] should have a corresponding
+/// [DataSink] for which it deserializes data.
+abstract class DataSource {
+  /// Deserialization of a section begin tag.
+  void begin(String tag);
+
+  /// Deserialization of a section end tag.
+  void end(String tag);
+
+  /// Deserialization of a string value.
+  String readString();
+
+  /// Deserialization of a non-negative integer value.
+  int readInt();
+
+  /// Deserialization of an enum value in [values].
+  E readEnum<E>(List<E> values);
+
+  /// Returns a string representation of the current state of the data source
+  /// useful for debugging in consistencies between serialization and
+  /// deserialization.
+  String get errorContext;
+}
diff --git a/pkg/compiler/lib/src/serialization/helpers.dart b/pkg/compiler/lib/src/serialization/helpers.dart
index 51e28e186a1..46791baac2c 100644
--- a/pkg/compiler/lib/src/serialization/helpers.dart
+++ b/pkg/compiler/lib/src/serialization/helpers.dart
@@ -166,66 +166,3 @@ class DartTypeNodeWriter
     visitTypes(node.typeArguments, functionTypeVariables);
   }
 }
-
-/// Data sink helper that canonicalizes [E] values using indices.
-class IndexedSink<E> {
-  final DataSink _sink;
-  Map<E, int> cache;
-
-  IndexedSink(this._sink, {this.cache}) {
-    // [cache] slot 0 is pre-allocated to `null`.
-    this.cache ??= {null: 0};
-  }
-
-  /// Write a reference to [value] to the data sink.
-  ///
-  /// If [value] has not been canonicalized yet, [writeValue] is called to
-  /// serialize the [value] itself.
-  void write(E value, void writeValue(E value)) {
-    const int pending = -1;
-    int index = cache[value];
-    if (index == null) {
-      index = cache.length;
-      _sink.writeInt(index);
-      cache[value] = pending; // Increments length to allocate slot.
-      writeValue(value);
-      cache[value] = index;
-    } else if (index == pending) {
-      throw ArgumentError("Cyclic dependency on cached value: $value");
-    } else {
-      _sink.writeInt(index);
-    }
-  }
-}
-
-/// Data source helper reads canonicalized [E] values through indices.
-class IndexedSource<E> {
-  final DataSource _sourceReader;
-  List<E> cache;
-
-  IndexedSource(this._sourceReader, {this.cache}) {
-    // [cache] slot 0 is pre-allocated to `null`.
-    this.cache ??= [null];
-  }
-
-  /// Reads a reference to an [E] value from the data source.
-  ///
-  /// If the value hasn't yet been read, [readValue] is called to deserialize
-  /// the value itself.
-  E read(E readValue()) {
-    int index = _sourceReader.readInt();
-    if (index >= cache.length) {
-      assert(index == cache.length);
-      cache.add(null); // placeholder.
-      E value = readValue();
-      cache[index] = value;
-      return value;
-    } else {
-      E value = cache[index];
-      if (value == null && index != 0) {
-        throw StateError('Unfilled index $index of $E');
-      }
-      return value;
-    }
-  }
-}
diff --git a/pkg/compiler/lib/src/serialization/indexed_sink_source.dart b/pkg/compiler/lib/src/serialization/indexed_sink_source.dart
new file mode 100644
index 00000000000..82db5e00010
--- /dev/null
+++ b/pkg/compiler/lib/src/serialization/indexed_sink_source.dart
@@ -0,0 +1,75 @@
+// Copyright (c) 2018, 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 'data_sink.dart';
+import 'data_source.dart';
+
+/// Data sink helper that canonicalizes [E?] values using indices.
+class IndexedSink<E extends Object> {
+  final DataSink _sink;
+  final Map<E?, int> cache;
+
+  IndexedSink._(this._sink, this.cache);
+
+  factory IndexedSink(DataSink sink, {Map<E?, int>? cache}) {
+    // [cache] slot 0 is pre-allocated to `null`.
+    cache ??= {null: 0};
+    return IndexedSink._(sink, cache);
+  }
+
+  /// Write a reference to [value] to the data sink.
+  ///
+  /// If [value] has not been canonicalized yet, [writeValue] is called to
+  /// serialize the [value] itself.
+  void write(E? value, void writeValue(E value)) {
+    const int pending = -1;
+    int? index = cache[value];
+    if (index == null) {
+      index = cache.length;
+      _sink.writeInt(index);
+      cache[value] = pending; // Increments length to allocate slot.
+      writeValue(value!); // `null` would have been found in slot 0.
+      cache[value] = index;
+    } else if (index == pending) {
+      throw ArgumentError("Cyclic dependency on cached value: $value");
+    } else {
+      _sink.writeInt(index);
+    }
+  }
+}
+
+/// Data source helper reads canonicalized [E?] values through indices.
+class IndexedSource<E extends Object> {
+  final DataSource _source;
+  final List<E?> cache;
+
+  IndexedSource._(this._source, this.cache);
+
+  factory IndexedSource(DataSource source, {List<E?>? cache}) {
+    // [cache] slot 0 is pre-allocated to `null`.
+    cache ??= [null];
+    return IndexedSource._(source, cache);
+  }
+
+  /// Reads a reference to an [E] value from the data source.
+  ///
+  /// If the value hasn't yet been read, [readValue] is called to deserialize
+  /// the value itself.
+  E? read(E readValue()) {
+    int index = _source.readInt();
+    if (index >= cache.length) {
+      assert(index == cache.length);
+      cache.add(null); // placeholder.
+      E value = readValue();
+      cache[index] = value;
+      return value;
+    } else {
+      E? value = cache[index];
+      if (value == null && index != 0) {
+        throw StateError('Unfilled index $index of $E');
+      }
+      return value;
+    }
+  }
+}
diff --git a/pkg/compiler/lib/src/serialization/object_sink.dart b/pkg/compiler/lib/src/serialization/object_sink.dart
index d5b54b4b5ef..3bbbd3b3765 100644
--- a/pkg/compiler/lib/src/serialization/object_sink.dart
+++ b/pkg/compiler/lib/src/serialization/object_sink.dart
@@ -2,45 +2,45 @@
 // 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.
 
-// @dart = 2.10
-
-part of 'serialization.dart';
+import 'data_sink.dart';
+import 'tags.dart' show Tag;
 
 /// [DataSinkWriter] that writes to a list of objects, useful for debugging
 /// inconsistencies between serialization and deserialization.
 ///
 /// This data sink writer works together with [ObjectDataSource].
 class ObjectDataSink implements DataSink {
-  List<dynamic> _data;
+  // [_data] is nullable and non-final to allow storage to be released.
+  List<dynamic>? _data;
 
   ObjectDataSink(this._data);
 
   @override
   void beginTag(String tag) {
-    _data.add(Tag('begin:$tag'));
+    _data!.add(Tag('begin:$tag'));
   }
 
   @override
   void endTag(String tag) {
-    _data.add(Tag('end:$tag'));
+    _data!.add(Tag('end:$tag'));
   }
 
   @override
   void writeEnum(dynamic value) {
-    assert(value != null);
-    _data.add(value);
+    assert((value as dynamic) != null); // TODO(48820): Remove when sound.
+    _data!.add(value);
   }
 
   @override
   void writeInt(int value) {
-    assert(value != null);
-    _data.add(value);
+    assert((value as dynamic) != null); // TODO(48820): Remove when sound.
+    _data!.add(value);
   }
 
   @override
   void writeString(String value) {
-    assert(value != null);
-    _data.add(value);
+    assert((value as dynamic) != null); // TODO(48820): Remove when sound.
+    _data!.add(value);
   }
 
   @override
@@ -50,5 +50,5 @@ class ObjectDataSink implements DataSink {
 
   /// Returns the number of objects written to this data sink.
   @override
-  int get length => _data.length;
+  int get length => _data!.length;
 }
diff --git a/pkg/compiler/lib/src/serialization/object_source.dart b/pkg/compiler/lib/src/serialization/object_source.dart
index bc349940458..f8279a2820b 100644
--- a/pkg/compiler/lib/src/serialization/object_source.dart
+++ b/pkg/compiler/lib/src/serialization/object_source.dart
@@ -2,9 +2,8 @@
 // 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.
 
-// @dart = 2.10
-
-part of 'serialization.dart';
+import 'data_source.dart';
+import 'tags.dart' show Tag;
 
 /// [DataSource] that read from a list of objects, useful for debugging
 /// inconsistencies between serialization and deserialization.
@@ -17,9 +16,9 @@ class ObjectDataSource implements DataSource {
   ObjectDataSource(this._data);
 
   T _read<T>() {
-    dynamic value = _data[_index++];
-    assert(value is T, "Expected $T value, found $value.$errorContext");
-    return value;
+    Object? value = _data[_index++];
+    if (value is T) return value;
+    throw StateError('Expected $T value, found $value.$errorContext');
   }
 
   @override
diff --git a/pkg/compiler/lib/src/serialization/serialization.dart b/pkg/compiler/lib/src/serialization/serialization.dart
index a863d059c7e..d0aa69e7e70 100644
--- a/pkg/compiler/lib/src/serialization/serialization.dart
+++ b/pkg/compiler/lib/src/serialization/serialization.dart
@@ -5,10 +5,8 @@
 // @dart = 2.10
 
 import 'dart:collection';
-import 'dart:convert';
 import 'dart:typed_data';
 import 'package:kernel/ast.dart' as ir;
-import 'package:kernel/binary/ast_to_binary.dart';
 import '../closure.dart';
 import '../constants/constant_system.dart' as constant_system;
 import '../constants/values.dart';
@@ -25,24 +23,24 @@ import '../js_model/closure.dart';
 import '../js_model/locals.dart';
 import '../js_model/type_recipe.dart' show TypeRecipe;
 
+import 'data_sink.dart';
+import 'data_source.dart';
 import 'member_data.dart';
-export 'member_data.dart' show ComponentLookup, computeMemberName;
 import 'serialization_interfaces.dart' as migrated
     show DataSourceReader, DataSinkWriter;
+import 'indexed_sink_source.dart';
 import 'tags.dart';
+
+export 'binary_sink.dart';
+export 'binary_source.dart';
+export 'member_data.dart' show ComponentLookup, computeMemberName;
+export 'object_sink.dart';
+export 'object_source.dart';
 export 'tags.dart';
 
 part 'sink.dart';
 part 'source.dart';
-part 'binary_sink.dart';
-part 'binary_source.dart';
 part 'helpers.dart';
-part 'object_sink.dart';
-part 'object_source.dart';
-
-abstract class StringInterner {
-  String internString(String string);
-}
 
 class ValueInterner {
   final Map<DartType, DartType> _dartTypeMap = HashMap();
diff --git a/pkg/compiler/lib/src/serialization/serialization_interfaces.dart b/pkg/compiler/lib/src/serialization/serialization_interfaces.dart
index a2ee9ddbd9b..efa9bb1ad82 100644
--- a/pkg/compiler/lib/src/serialization/serialization_interfaces.dart
+++ b/pkg/compiler/lib/src/serialization/serialization_interfaces.dart
@@ -9,6 +9,10 @@ import '../elements/types.dart' show DartType;
 
 export 'tags.dart';
 
+abstract class StringInterner {
+  String internString(String string);
+}
+
 /// NNBD-migrated interface for methods of DataSinkWriter.
 ///
 /// This is a pure interface or facade for DataSinkWriter.
@@ -73,6 +77,7 @@ abstract class DataSinkWriter {
       {bool allowNull = false});
 
   void writeCached<E extends Object>(E value, void f(E value));
+
   void writeList<E extends Object>(Iterable<E> values, void f(E value),
       {bool allowNull = false});
 }
@@ -112,6 +117,7 @@ abstract class DataSourceReader {
   Map<K, V>? readTreeNodeMapInContextOrNull<K extends ir.TreeNode, V>(V f());
 
   E readCached<E extends Object>(E f());
+
   List<E> readList<E extends Object>(E f());
   List<E>? readListOrNull<E extends Object>(E f());
 }
diff --git a/pkg/compiler/lib/src/serialization/sink.dart b/pkg/compiler/lib/src/serialization/sink.dart
index 67ec6b5cdcb..214b6e9f413 100644
--- a/pkg/compiler/lib/src/serialization/sink.dart
+++ b/pkg/compiler/lib/src/serialization/sink.dart
@@ -6,32 +6,6 @@
 
 part of 'serialization.dart';
 
-/// Interface handling [DataSinkWriter] low-level data serialization.
-///
-/// Each implementation of [DataSink] should have a corresponding
-/// [DataSource] that deserializes data serialized by that implementation.
-abstract class DataSink {
-  int get length;
-
-  /// Serialization of a non-negative integer value.
-  void writeInt(int value);
-
-  /// Serialization of an enum value.
-  void writeEnum(dynamic value);
-
-  /// Serialization of a String value.
-  void writeString(String value);
-
-  /// Serialization of a section begin tag. May be omitted by some writers.
-  void beginTag(String tag);
-
-  /// Serialization of a section end tag. May be omitted by some writers.
-  void endTag(String tag);
-
-  /// Closes any underlying data sinks.
-  void close();
-}
-
 /// Serialization writer
 ///
 /// To be used with [DataSourceReader] to read and write serialized data.
diff --git a/pkg/compiler/lib/src/serialization/source.dart b/pkg/compiler/lib/src/serialization/source.dart
index 806731c75f1..2d0855ceecb 100644
--- a/pkg/compiler/lib/src/serialization/source.dart
+++ b/pkg/compiler/lib/src/serialization/source.dart
@@ -6,32 +6,6 @@
 
 part of 'serialization.dart';
 
-/// Interface handling [DataSourceReader] low-level data deserialization.
-///
-/// Each implementation of [DataSource] should have a corresponding
-/// [DataSink] for which it deserializes data.
-abstract class DataSource {
-  /// Deserialization of a section begin tag.
-  void begin(String tag);
-
-  /// Deserialization of a section end tag.
-  void end(String tag);
-
-  /// Deserialization of a string value.
-  String readString();
-
-  /// Deserialization of a non-negative integer value.
-  int readInt();
-
-  /// Deserialization of an enum value in [values].
-  E readEnum<E>(List<E> values);
-
-  /// Returns a string representation of the current state of the data source
-  /// useful for debugging in consistencies between serialization and
-  /// deserialization.
-  String get errorContext;
-}
-
 /// Deserialization reader
 ///
 /// To be used with [DataSinkWriter] to read and write serialized data.
diff --git a/pkg/compiler/lib/src/serialization/task.dart b/pkg/compiler/lib/src/serialization/task.dart
index 6f9d2c46f02..b4e8b0b6ec4 100644
--- a/pkg/compiler/lib/src/serialization/task.dart
+++ b/pkg/compiler/lib/src/serialization/task.dart
@@ -29,6 +29,7 @@ import '../options.dart';
 import '../util/sink_adapter.dart';
 import '../world.dart';
 import 'serialization.dart';
+import 'serialization_interfaces.dart' show StringInterner;
 
 /// A data class holding a [JsClosedWorld] and the associated
 /// [DataSourceIndices].