Port package to Dart 2.0

[matejm]

When building example in Dart 2.0, I get following error:

I/FlutterActivityDelegate(17819): onResume setting current activity to this
E/flutter (17819): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter (17819): type 'Future<dynamic>' is not a subtype of type 'Future<bool>' where
E/flutter (17819):   Future is from dart:async
E/flutter (17819):   Future is from dart:async
E/flutter (17819):   bool is from dart:core
E/flutter (17819): 
E/flutter (17819): #0      Screen.isKeptOn (package:screen/screen.dart:10:48)
E/flutter (17819): #1      _MyAppState.initPlatformState (file:///home/matejm/Desktop/flutter_screen/example/lib/main.dart:22:32)
E/flutter (17819): <asynchronous suspension>
E/flutter (17819): #2      _MyAppState.initState (file:///home/matejm/Desktop/flutter_screen/example/lib/main.dart:18:5)
E/flutter (17819): #3      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3743:58)
E/flutter (17819): #4      ComponentElement.mount (package:flutter/src/widgets/framework.dart:3609:5)
E/flutter (17819): #5      Element.inflateWidget (package:flutter/src/widgets/framework.dart:2899:14)
E/flutter (17819): #6      Element.updateChild (package:flutter/src/widgets/framework.dart:2702:12)
E/flutter (17819): #7      RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:857:16)
E/flutter (17819): #8      RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:828:5)
E/flutter (17819): #9      RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:774:17)
E/flutter (17819): #10     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2214:19)
E/flutter (17819): #11     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:773:13)
E/flutter (17819): #12     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&RendererBinding&WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:662:7)
E/flutter (17819): #13     runApp (package:flutter/src/widgets/binding.dart:704:7)
E/flutter (17819): #14     main (file:///home/matejm/Desktop/flutter_screen/example/lib/main.dart:4:16)
E/flutter (17819): #15     _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:279:19)
E/flutter (17819): #16     _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:165:12)

It is probably because Dart 2.0 introduces strong types, which do not allow implicit casting.

I am going to create a pull request that fixes this problem (at least for me).