diff --git a/packages/flutter/lib/src/services/message_codec.dart b/packages/flutter/lib/src/services/message_codec.dart
index 063fa75a80..394f6a65f0 100644
--- a/packages/flutter/lib/src/services/message_codec.dart
+++ b/packages/flutter/lib/src/services/message_codec.dart
@@ -29,7 +29,7 @@ abstract class MessageCodec<T> {
   /// Decodes the specified [message] from binary.
   ///
   /// Returns null if the message is null.
-  T decodeMessage(ByteData? message);
+  T? decodeMessage(ByteData? message);
 }
 
 /// An command object representing the invocation of a named method.
@@ -73,16 +73,22 @@ abstract class MethodCodec {
   ///
   /// Throws [PlatformException], if [envelope] represents an error, otherwise
   /// returns the enveloped result.
+  ///
+  /// The type returned from [decodeEnvelope] is `dynamic` (not `Object?`),
+  /// which means *no type checking is performed on its return value*. It is
+  /// strongly recommended that the return value be immediately cast to a known
+  /// type to prevent runtime errors due to typos that the type checker could
+  /// otherwise catch.
   dynamic decodeEnvelope(ByteData envelope);
 
   /// Encodes a successful [result] into a binary envelope.
-  ByteData encodeSuccessEnvelope(dynamic result);
+  ByteData encodeSuccessEnvelope(Object? result);
 
   /// Encodes an error result into a binary envelope.
   ///
   /// The specified error [code], human-readable error [message] and error
   /// [details] correspond to the fields of [PlatformException].
-  ByteData encodeErrorEnvelope({ required String code, String? message, dynamic details});
+  ByteData encodeErrorEnvelope({ required String code, String? message, Object? details});
 }
 
 
@@ -117,7 +123,7 @@ class PlatformException implements Exception {
   final String? message;
 
   /// Error details, possibly null.
-  final dynamic details;
+  final Object? details;
 
   /// Native stacktrace for the error, possibly null.
   /// This is strictly for native platform stacktrace.
diff --git a/packages/flutter/lib/src/services/message_codecs.dart b/packages/flutter/lib/src/services/message_codecs.dart
index 92f9d722e8..cb5e4ac118 100644
--- a/packages/flutter/lib/src/services/message_codecs.dart
+++ b/packages/flutter/lib/src/services/message_codecs.dart
@@ -17,7 +17,7 @@ import 'message_codec.dart';
 /// When sending outgoing messages from Android, be sure to use direct `ByteBuffer`
 /// as opposed to indirect. The `wrap()` API provides indirect buffers by default
 /// and you will get empty `ByteData` objects in Dart.
-class BinaryCodec implements MessageCodec<ByteData?> {
+class BinaryCodec implements MessageCodec<ByteData> {
   /// Creates a [MessageCodec] with unencoded binary messages represented using
   /// [ByteData].
   const BinaryCodec();
@@ -33,7 +33,7 @@ class BinaryCodec implements MessageCodec<ByteData?> {
 ///
 /// On Android, messages will be represented using `java.util.String`.
 /// On iOS, messages will be represented using `NSString`.
-class StringCodec implements MessageCodec<String?> {
+class StringCodec implements MessageCodec<String> {
   /// Creates a [MessageCodec] with UTF-8 encoded String messages.
   const StringCodec();
 
@@ -71,7 +71,13 @@ class StringCodec implements MessageCodec<String?> {
 /// null/nil for null, and identical to what would result from decoding a
 /// singleton JSON array with a Boolean, number, or string value, and then
 /// extracting its single element.
-class JSONMessageCodec implements MessageCodec<dynamic> {
+///
+/// The type returned from [decodeMessage] is `dynamic` (not `Object?`), which
+/// means *no type checking is performed on its return value*. It is strongly
+/// recommended that the return value be immediately cast to a known type to
+/// prevent runtime errors due to typos that the type checker could otherwise
+/// catch.
+class JSONMessageCodec implements MessageCodec<Object?> {
   // The codec serializes messages as defined by the JSON codec of the
   // dart:convert package. The format used must match the Android and
   // iOS counterparts.
@@ -80,7 +86,7 @@ class JSONMessageCodec implements MessageCodec<dynamic> {
   const JSONMessageCodec();
 
   @override
-  ByteData? encodeMessage(dynamic message) {
+  ByteData? encodeMessage(Object? message) {
     if (message == null)
       return null;
     return const StringCodec().encodeMessage(json.encode(message));
@@ -118,7 +124,7 @@ class JSONMethodCodec implements MethodCodec {
 
   @override
   ByteData encodeMethodCall(MethodCall call) {
-    return const JSONMessageCodec().encodeMessage(<String, dynamic>{
+    return const JSONMessageCodec().encodeMessage(<String, Object?>{
       'method': call.method,
       'args': call.arguments,
     })!;
@@ -126,11 +132,11 @@ class JSONMethodCodec implements MethodCodec {
 
   @override
   MethodCall decodeMethodCall(ByteData? methodCall) {
-    final dynamic decoded = const JSONMessageCodec().decodeMessage(methodCall);
+    final Object? decoded = const JSONMessageCodec().decodeMessage(methodCall);
     if (decoded is! Map)
       throw FormatException('Expected method call Map, got $decoded');
-    final dynamic method = decoded['method'];
-    final dynamic arguments = decoded['args'];
+    final Object? method = decoded['method'];
+    final Object? arguments = decoded['args'];
     if (method is String)
       return MethodCall(method, arguments);
     throw FormatException('Invalid method call: $decoded');
@@ -138,7 +144,7 @@ class JSONMethodCodec implements MethodCodec {
 
   @override
   dynamic decodeEnvelope(ByteData envelope) {
-    final dynamic decoded = const JSONMessageCodec().decodeMessage(envelope);
+    final Object? decoded = const JSONMessageCodec().decodeMessage(envelope);
     if (decoded is! List)
       throw FormatException('Expected envelope List, got $decoded');
     if (decoded.length == 1)
@@ -165,14 +171,14 @@ class JSONMethodCodec implements MethodCodec {
   }
 
   @override
-  ByteData encodeSuccessEnvelope(dynamic result) {
-    return const JSONMessageCodec().encodeMessage(<dynamic>[result])!;
+  ByteData encodeSuccessEnvelope(Object? result) {
+    return const JSONMessageCodec().encodeMessage(<Object?>[result])!;
   }
 
   @override
-  ByteData encodeErrorEnvelope({ required String code, String? message, dynamic details}) {
+  ByteData encodeErrorEnvelope({ required String code, String? message, Object? details}) {
     assert(code != null);
-    return const JSONMessageCodec().encodeMessage(<dynamic>[code, message, details])!;
+    return const JSONMessageCodec().encodeMessage(<Object?>[code, message, details])!;
   }
 }
 
@@ -188,9 +194,20 @@ class JSONMethodCodec implements MethodCodec {
 ///  * [List]s of supported values
 ///  * [Map]s from supported values to supported values
 ///
-/// Decoded values will use `List<dynamic>` and `Map<dynamic, dynamic>`
+/// Decoded values will use `List<Object?>` and `Map<Object?, Object?>`
 /// irrespective of content.
 ///
+/// The type returned from [decodeMessage] is `dynamic` (not `Object?`), which
+/// means *no type checking is performed on its return value*. It is strongly
+/// recommended that the return value be immediately cast to a known type to
+/// prevent runtime errors due to typos that the type checker could otherwise
+/// catch.
+///
+/// The codec is extensible by subclasses overriding [writeValue] and
+/// [readValueOfType].
+///
+/// ## Android specifics
+///
 /// On Android, messages are represented as follows:
 ///
 ///  * null: null
@@ -206,6 +223,14 @@ class JSONMethodCodec implements MethodCodec {
 ///  * [List]\: `java.util.ArrayList`
 ///  * [Map]\: `java.util.HashMap`
 ///
+/// When sending a `java.math.BigInteger` from Java, it is converted into a
+/// [String] with the hexadecimal representation of the integer. (The value is
+/// tagged as being a big integer; subclasses of this class could be made to
+/// support it natively; see the discussion at [writeValue].) This codec does
+/// not support sending big integers from Dart.
+///
+/// ## iOS specifics
+///
 /// On iOS, messages are represented as follows:
 ///
 ///  * null: nil
@@ -218,16 +243,7 @@ class JSONMethodCodec implements MethodCodec {
 ///    `FlutterStandardTypedData`
 ///  * [List]\: `NSArray`
 ///  * [Map]\: `NSDictionary`
-///
-/// When sending a `java.math.BigInteger` from Java, it is converted into a
-/// [String] with the hexadecimal representation of the integer. (The value is
-/// tagged as being a big integer; subclasses of this class could be made to
-/// support it natively; see the discussion at [writeValue].) This codec does
-/// not support sending big integers from Dart.
-///
-/// The codec is extensible by subclasses overriding [writeValue] and
-/// [readValueOfType].
-class StandardMessageCodec implements MessageCodec<dynamic> {
+class StandardMessageCodec implements MessageCodec<Object?> {
   /// Creates a [MessageCodec] using the Flutter standard binary encoding.
   const StandardMessageCodec();
 
@@ -289,7 +305,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   static const int _valueMap = 13;
 
   @override
-  ByteData? encodeMessage(dynamic message) {
+  ByteData? encodeMessage(Object? message) {
     if (message == null)
       return null;
     final WriteBuffer buffer = WriteBuffer();
@@ -302,7 +318,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
     if (message == null)
       return null;
     final ReadBuffer buffer = ReadBuffer(message);
-    final dynamic result = readValue(buffer);
+    final Object? result = readValue(buffer);
     if (buffer.hasRemaining)
       throw const FormatException('Message corrupted');
     return result;
@@ -344,7 +360,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   /// string's length as encoded by [writeSize] followed by the string bytes. On
   /// Android, that would get converted to a `java.math.BigInteger` object. On
   /// iOS, the string representation is returned.
-  void writeValue(WriteBuffer buffer, dynamic value) {
+  void writeValue(WriteBuffer buffer, Object? value) {
     if (value == null) {
       buffer.putUint8(_valueNull);
     } else if (value is bool) {
@@ -389,13 +405,13 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
     } else if (value is List) {
       buffer.putUint8(_valueList);
       writeSize(buffer, value.length);
-      for (final dynamic item in value) {
+      for (final Object? item in value) {
         writeValue(buffer, item);
       }
     } else if (value is Map) {
       buffer.putUint8(_valueMap);
       writeSize(buffer, value.length);
-      value.forEach((dynamic key, dynamic value) {
+      value.forEach((Object? key, Object? value) {
         writeValue(buffer, key);
         writeValue(buffer, value);
       });
@@ -408,7 +424,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   ///
   /// This method is intended for use by subclasses overriding
   /// [readValueOfType].
-  dynamic readValue(ReadBuffer buffer) {
+  Object? readValue(ReadBuffer buffer) {
     if (!buffer.hasRemaining)
       throw const FormatException('Message corrupted');
     final int type = buffer.getUint8();
@@ -420,7 +436,7 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   /// The codec can be extended by overriding this method, calling super for
   /// types that the extension does not handle. See the discussion at
   /// [writeValue].
-  dynamic readValueOfType(int type, ReadBuffer buffer) {
+  Object? readValueOfType(int type, ReadBuffer buffer) {
     switch (type) {
       case _valueNull:
         return null;
@@ -452,13 +468,13 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
         return buffer.getFloat64List(length);
       case _valueList:
         final int length = readSize(buffer);
-        final List<dynamic> result = List<dynamic>.filled(length, null, growable: false);
+        final List<Object?> result = List<Object?>.filled(length, null, growable: false);
         for (int i = 0; i < length; i++)
           result[i] = readValue(buffer);
         return result;
       case _valueMap:
         final int length = readSize(buffer);
-        final Map<dynamic, dynamic> result = <dynamic, dynamic>{};
+        final Map<Object?, Object?> result = <Object?, Object?>{};
         for (int i = 0; i < length; i++)
           result[readValue(buffer)] = readValue(buffer);
         return result;
@@ -539,8 +555,8 @@ class StandardMethodCodec implements MethodCodec {
   @override
   MethodCall decodeMethodCall(ByteData? methodCall) {
     final ReadBuffer buffer = ReadBuffer(methodCall!);
-    final dynamic method = messageCodec.readValue(buffer);
-    final dynamic arguments = messageCodec.readValue(buffer);
+    final Object? method = messageCodec.readValue(buffer);
+    final Object? arguments = messageCodec.readValue(buffer);
     if (method is String && !buffer.hasRemaining)
       return MethodCall(method, arguments);
     else
@@ -548,7 +564,7 @@ class StandardMethodCodec implements MethodCodec {
   }
 
   @override
-  ByteData encodeSuccessEnvelope(dynamic result) {
+  ByteData encodeSuccessEnvelope(Object? result) {
     final WriteBuffer buffer = WriteBuffer();
     buffer.putUint8(0);
     messageCodec.writeValue(buffer, result);
@@ -556,7 +572,7 @@ class StandardMethodCodec implements MethodCodec {
   }
 
   @override
-  ByteData encodeErrorEnvelope({ required String code, String? message, dynamic details}) {
+  ByteData encodeErrorEnvelope({ required String code, String? message, Object? details}) {
     final WriteBuffer buffer = WriteBuffer();
     buffer.putUint8(1);
     messageCodec.writeValue(buffer, code);
@@ -573,10 +589,10 @@ class StandardMethodCodec implements MethodCodec {
     final ReadBuffer buffer = ReadBuffer(envelope);
     if (buffer.getUint8() == 0)
       return messageCodec.readValue(buffer);
-    final dynamic errorCode = messageCodec.readValue(buffer);
-    final dynamic errorMessage = messageCodec.readValue(buffer);
-    final dynamic errorDetails = messageCodec.readValue(buffer);
-    final String? errorStacktrace = (buffer.hasRemaining) ? messageCodec.readValue(buffer) as String : null;
+    final Object? errorCode = messageCodec.readValue(buffer);
+    final Object? errorMessage = messageCodec.readValue(buffer);
+    final Object? errorDetails = messageCodec.readValue(buffer);
+    final String? errorStacktrace = (buffer.hasRemaining) ? messageCodec.readValue(buffer) as String? : null;
     if (errorCode is String && (errorMessage == null || errorMessage is String) && !buffer.hasRemaining)
       throw PlatformException(code: errorCode, message: errorMessage as String?, details: errorDetails, stacktrace: errorStacktrace);
     else
diff --git a/packages/flutter/lib/src/services/platform_channel.dart b/packages/flutter/lib/src/services/platform_channel.dart
index 3f176c3929..3f9383e9f1 100644
--- a/packages/flutter/lib/src/services/platform_channel.dart
+++ b/packages/flutter/lib/src/services/platform_channel.dart
@@ -52,7 +52,7 @@ class BasicMessageChannel<T> {
   ///
   /// Returns a [Future] which completes to the received response, which may
   /// be null.
-  Future<T> send(T message) async {
+  Future<T?> send(T message) async {
     return codec.decodeMessage(await binaryMessenger.send(name, codec.encodeMessage(message)));
   }
 
@@ -65,7 +65,7 @@ class BasicMessageChannel<T> {
   ///
   /// The handler's return value is sent back to the platform plugins as a
   /// message reply. It may be null.
-  void setMessageHandler(Future<T> Function(T message)? handler) {
+  void setMessageHandler(Future<T> Function(T? message)? handler) {
     if (handler == null) {
       binaryMessenger.setMessageHandler(name, null);
     } else {
@@ -86,7 +86,7 @@ class BasicMessageChannel<T> {
   ///
   /// This is intended for testing. Messages intercepted in this manner are not
   /// sent to platform plugins.
-  void setMockMessageHandler(Future<T> Function(T message)? handler) {
+  void setMockMessageHandler(Future<T> Function(T? message)? handler) {
     if (handler == null) {
       binaryMessenger.setMockMessageHandler(name, null);
     } else {
diff --git a/packages/flutter/lib/src/services/restoration.dart b/packages/flutter/lib/src/services/restoration.dart
index 97b991e9df..22f9ba90d7 100644
--- a/packages/flutter/lib/src/services/restoration.dart
+++ b/packages/flutter/lib/src/services/restoration.dart
@@ -226,7 +226,7 @@ class RestorationManager extends ChangeNotifier {
   bool _isReplacing = false;
 
   Future<void> _getRootBucketFromEngine() async {
-    final Map<dynamic, dynamic>? config = await SystemChannels.restoration.invokeMethod<Map<dynamic, dynamic>>('get');
+    final Map<Object?, Object?>? config = await SystemChannels.restoration.invokeMethod<Map<Object?, Object?>>('get');
     if (_pendingRootBucket == null) {
       // The restoration data was obtained via other means (e.g. by calling
       // [handleRestorationDataUpdate] while the request to the engine was
@@ -237,9 +237,9 @@ class RestorationManager extends ChangeNotifier {
     _parseAndHandleRestorationUpdateFromEngine(config);
   }
 
-  void _parseAndHandleRestorationUpdateFromEngine(Map<dynamic, dynamic>? update) {
+  void _parseAndHandleRestorationUpdateFromEngine(Map<Object?, Object?>? update) {
     handleRestorationUpdateFromEngine(
-      enabled: update != null && update['enabled'] as bool,
+      enabled: update != null && update['enabled']! as bool,
       data: update == null ? null : update['data'] as Uint8List?,
     );
   }
@@ -305,25 +305,25 @@ class RestorationManager extends ChangeNotifier {
     );
   }
 
-  Future<dynamic> _methodHandler(MethodCall call) async {
+  Future<Object?> _methodHandler(MethodCall call) async {
     switch (call.method) {
       case 'push':
-        _parseAndHandleRestorationUpdateFromEngine(call.arguments as Map<dynamic, dynamic>);
+        _parseAndHandleRestorationUpdateFromEngine(call.arguments as Map<Object?, Object?>);
         break;
       default:
         throw UnimplementedError("${call.method} was invoked but isn't implemented by $runtimeType");
     }
   }
 
-  Map<dynamic, dynamic>? _decodeRestorationData(Uint8List? data) {
+  Map<Object?, Object?>? _decodeRestorationData(Uint8List? data) {
     if (data == null) {
       return null;
     }
     final ByteData encoded = data.buffer.asByteData(data.offsetInBytes, data.lengthInBytes);
-    return const StandardMessageCodec().decodeMessage(encoded) as Map<dynamic, dynamic>;
+    return const StandardMessageCodec().decodeMessage(encoded) as Map<Object?, Object?>?;
   }
 
-  Uint8List _encodeRestorationData(Map<dynamic, dynamic> data) {
+  Uint8List _encodeRestorationData(Map<Object?, Object?> data) {
     final ByteData encoded = const StandardMessageCodec().encodeMessage(data)!;
     return encoded.buffer.asUint8List(encoded.offsetInBytes, encoded.lengthInBytes);
   }
@@ -505,7 +505,7 @@ class RestorationBucket {
     required Object? debugOwner,
   }) : assert(restorationId != null),
        _restorationId = restorationId,
-       _rawData = <String, dynamic>{} {
+       _rawData = <String, Object?>{} {
     assert(() {
       _debugOwner = debugOwner;
       return true;
@@ -537,10 +537,10 @@ class RestorationBucket {
   /// The `manager` argument must not be null.
   RestorationBucket.root({
     required RestorationManager manager,
-    required Map<dynamic, dynamic>? rawData,
+    required Map<Object?, Object?>? rawData,
   }) : assert(manager != null),
        _manager = manager,
-       _rawData = rawData ?? <dynamic, dynamic>{},
+       _rawData = rawData ?? <Object?, Object?>{},
        _restorationId = 'root' {
     assert(() {
       _debugOwner = manager;
@@ -567,7 +567,7 @@ class RestorationBucket {
        assert(parent._rawChildren[restorationId] != null),
        _manager = parent._manager,
        _parent = parent,
-       _rawData = parent._rawChildren[restorationId] as Map<dynamic, dynamic>,
+       _rawData = parent._rawChildren[restorationId]! as Map<Object?, Object?>,
        _restorationId = restorationId {
     assert(() {
       _debugOwner = debugOwner;
@@ -578,7 +578,7 @@ class RestorationBucket {
   static const String _childrenMapKey = 'c';
   static const String _valuesMapKey = 'v';
 
-  final Map<dynamic, dynamic> _rawData;
+  final Map<Object?, Object?> _rawData;
 
   /// The owner of the bucket that was provided when the bucket was claimed via
   /// [claimChild].
@@ -616,9 +616,9 @@ class RestorationBucket {
   String _restorationId;
 
   // Maps a restoration ID to the raw map representation of a child bucket.
-  Map<dynamic, dynamic> get _rawChildren => _rawData.putIfAbsent(_childrenMapKey, () => <dynamic, dynamic>{}) as Map<dynamic, dynamic>;
+  Map<Object?, Object?> get _rawChildren => _rawData.putIfAbsent(_childrenMapKey, () => <Object?, Object?>{})! as Map<Object?, Object?>;
   // Maps a restoration ID to a value that is stored in this bucket.
-  Map<dynamic, dynamic> get _rawValues => _rawData.putIfAbsent(_valuesMapKey, () => <dynamic, dynamic>{}) as Map<dynamic, dynamic>;
+  Map<Object?, Object?> get _rawValues => _rawData.putIfAbsent(_valuesMapKey, () => <Object?, Object?>{})! as Map<Object?, Object?>;
 
   // Get and store values.
 
diff --git a/packages/flutter/test/services/message_codecs_testing.dart b/packages/flutter/test/services/message_codecs_testing.dart
index 26e3edd298..5c561bcf50 100644
--- a/packages/flutter/test/services/message_codecs_testing.dart
+++ b/packages/flutter/test/services/message_codecs_testing.dart
@@ -17,13 +17,13 @@ void checkEncoding<T>(MessageCodec<T> codec, T message, List<int> expectedBytes)
 
 void checkEncodeDecode<T>(MessageCodec<T> codec, T message) {
   final ByteData? encoded = codec.encodeMessage(message);
-  final T decoded = codec.decodeMessage(encoded);
+  final T? decoded = codec.decodeMessage(encoded);
   if (message == null) {
     expect(encoded, isNull);
     expect(decoded, isNull);
   } else {
     expect(deepEquals(message, decoded), isTrue);
-    final ByteData? encodedAgain = codec.encodeMessage(decoded);
+    final ByteData? encodedAgain = codec.encodeMessage(decoded as T);
     expect(
       encodedAgain!.buffer.asUint8List(),
       orderedEquals(encoded!.buffer.asUint8List()),
diff --git a/packages/flutter_web_plugins/test/plugin_registry_test.dart b/packages/flutter_web_plugins/test/plugin_registry_test.dart
index 7b950d7fdd..a38fb23bdc 100644
--- a/packages/flutter_web_plugins/test/plugin_registry_test.dart
+++ b/packages/flutter_web_plugins/test/plugin_registry_test.dart
@@ -56,7 +56,7 @@ void main() {
       final List<String> loggedMessages = <String>[];
       ServicesBinding.instance!.defaultBinaryMessenger
           .setMessageHandler('test_send', (ByteData? data) {
-        loggedMessages.add(codec.decodeMessage(data) as String);
+        loggedMessages.add(codec.decodeMessage(data)! as String);
         return Future<ByteData?>.value(null);
       });