Dart 2 compability

[philipgiuliani]

The library does not work properly with Dart 2.

Flutter's Dart 1 support will be dropped in Beta 3 (https://github.com/flutter/flutter/wiki/Opting-out-of-Dart-2-with-Flutter-beta-2)

E/flutter ( 5081): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 5081): type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'String' of 'message' where
E/flutter ( 5081):   _InternalLinkedHashMap is from dart:collection
E/flutter ( 5081):   String is from dart:core
E/flutter ( 5081):   String is from dart:core

E/flutter ( 5081): #1      PhoenixPush.matchReceive.<anonymous closure> (package:phoenix_wings/src/phoenix_push.dart:38)
E/flutter ( 5081): #2      Iterable.forEach (dart:core/iterable.dart:292)
E/flutter ( 5081): #3      PhoenixPush.matchReceive (package:phoenix_wings/src/phoenix_push.dart:38)
E/flutter ( 5081): #4      PhoenixPush.startTimeout.<anonymous closure> (package:phoenix_wings/src/phoenix_push.dart:78)
E/flutter ( 5081): #5      PhoenixChannel.trigger.<anonymous closure> (package:phoenix_wings/src/phoenix_channel.dart:204)
E/flutter ( 5081): #6      Iterable.forEach (dart:core/iterable.dart:292)
E/flutter ( 5081): #7      PhoenixChannel.trigger (package:phoenix_wings/src/phoenix_channel.dart:203)
E/flutter ( 5081): #8      new PhoenixChannel.<anonymous closure> (package:phoenix_wings/src/phoenix_channel.dart:73)
E/flutter ( 5081): #9      PhoenixChannel.trigger.<anonymous closure> (package:phoenix_wings/src/phoenix_channel.dart:204)
E/flutter ( 5081): #10     Iterable.forEach (dart:core/iterable.dart:292)
E/flutter ( 5081): #11     PhoenixChannel.trigger (package:phoenix_wings/src/phoenix_channel.dart:203)
E/flutter ( 5081): #12     PhoenixSocket._onConnMessage.<anonymous closure> (package:phoenix_wings/src/phoenix_socket.dart:134)
E/flutter ( 5081): #13     Iterable.forEach (dart:core/iterable.dart:292)
E/flutter ( 5081): #14     PhoenixSocket._onConnMessage (package:phoenix_wings/src/phoenix_socket.dart:134)

The crash is caused by the JSON libary. See "What is breaking?": https://groups.google.com/forum/#!topic/flutter-dev/H8dDhWg_c8I