StandardMessageCodec produces Map<dynamic, dynamic> but users expect Map<String, dynamic>

[mraleph]

Consider for example this code from semantics_service_test (/cc @amirh )

    SystemChannels.accessibility.setMockMessageHandler((Object mockMessage) async {
      final Map<String, dynamic> message = mockMessage;
      log.add(message);
    });

This triggers runtime check failure in Dart 2 mode because Map<dynamic, dynamic> is not a subtype of Map<String, dynamic>.

I have discussed this with @mravn-google and we decided to go by fixing the consumer side, especially because Dart lacks features needed to correctly preserve type arguments across channel boundaries for arbitrary types.

I am gonna handle updating tests in Flutter repo, while @mravn-google team is gonna take care of packages that contain similar issues, e.g. https://github.com/flutter/plugins/blob/master/packages/video_player/lib/video_player.dart#L162-L163

[a-siva]

In addition to the above I also ran into the following runtime errors which needed to be fixed before the video demo comes up in flutter gallery

E/flutter ( 5722): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 5722): type '_BroadcastSubscription' is not a subtype of type 'StreamSubscription<Map<String, dynamic>>' where
E/flutter ( 5722):   _BroadcastSubscription is from dart:async
E/flutter ( 5722):   StreamSubscription is from dart:async
E/flutter ( 5722):   Map is from dart:core
E/flutter ( 5722):   String is from dart:core
E/flutter ( 5722): 
E/flutter ( 5722): #0      VideoPlayerController.initialize (package:video_player/video_player.dart:191:10)
E/flutter ( 5722): <asynchronous suspension>
E/flutter ( 5722): #1      _VideoDemoState.initState.initController (file:///usr/local/google/home/asiva/workspace/flutter-roll/flutter/examples/flutter_gallery/lib/demo/video_demo.dart:385:24)
E/flutter ( 5722): <asynchronous suspension>
E/flutter ( 5722): #2      _VideoDemoState.initState (file:///usr/local/google/home/asiva/workspace/flutter-roll/flutter/examples/flutter_gallery/lib/demo/video

/flutter ( 5434): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 5434): type 'List' is not a subtype of type 'List<List<int>>' where
E/flutter ( 5434):   List is from dart:core
E/flutter ( 5434):   List is from dart:core
E/flutter ( 5434):   List is from dart:core
E/flutter ( 5434):   int is from dart:core
E/flutter ( 5434):
E/flutter ( 5434): #0      VideoPlayerController.initialize.eventListener (package:video_player/video_player.dart:176:51)
E/flutter ( 5434): #1      _RootZone.runUnaryGuarded (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/zone.dart:1316:10)
E/flutter ( 5434): #2      _BufferingStreamSubscription._sendData (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_impl.dart:330:11)

E/flutter ( 3775): type '(PlatformException) => void' is not a subtype of type '(Object) => FutureOr'
E/flutter ( 3775): #0      _registerErrorHandler (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/async_error.dart:22:60)
E/flutter ( 3775): #1      _BufferingStreamSubscription.onError (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_impl.dart:146:16)
E/flutter ( 3775): #2      new _BufferingStreamSubscription (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_impl.dart:113:10)
E/flutter ( 3775): #3      new _ControllerSubscription (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_controller.dart)
E/flutter ( 3775): #4      new _BroadcastSubscription (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/broadcast_stream_controller.dart)
E/flutter ( 3775): #5      _BroadcastStreamController._subscribe (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/broadcast_stream_controller.dart:212:46)
E/flutter ( 3775): #6      _ControllerStream._createSubscription (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_controller.dart:817:19)
E/flutter ( 3775): #7      _StreamImpl.listen (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_impl.dart:466:9)
E/flutter ( 3775): #8      VideoPlayerController.initialize (package:video_player/video_player.dart:190:10)

D/MediaPlayer( 5722): setSubtitleAnchor in MediaPlayer
E/flutter ( 5722): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 5722): 'package:video_player/video_player.dart': error: line 178 pos 40: file:///usr/local/google/home/asiva/.pub-cache/hosted/pub.dartlang.org/video_player-0.2.0/lib/video_player.dart:178:40: Error: A value of type '(dart.core::List<dart.core::int>) → #lib1::DurationRange' can't be assigned to a variable of type '(dynamic) → dynamic'.
E/flutter ( 5722): Try changing the type of the left hand side, or casting the right hand side to '(dynamic) → dynamic'.
E/flutter ( 5722):           buffered: bufferedValues.map(toDurationRange).toList(),
E/flutter ( 5722):                                        ^
E/flutter ( 5722):           buffered: bufferedValues.map(toDurationRange).toList(),
E/flutter ( 5722):                                        ^
E/flutter ( 5722): 
E/flutter ( 5722): #0      _RootZone.runUnaryGuarded (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/zone.dart:1316:10)
E/flutter ( 5722): #1      _BufferingStreamSubscription._sendData (file:///b/build/slave/Linux_Engine/build/src/third_party/dart/sdk/lib/async/stream_impl.dart

Once I worked around these errors by doing local patches in
.pub-cache/hosted/pub.dartlang.org/video_player-0.2.0/lib/video_player.dart
The video demo comes up and works.

[mraleph]

@a-siva would you make PR with your changes - given that you already have it fixed?

[a-siva]

created PR flutter/plugins#380 for addressing these issues