Dart 2 runtime failures in tests

[keertip]

Running the tests in dart 2 mode, see failures

type 'Future' is not a subtype of type 'Future' where
Future is from dart:async
Future is from dart:async
String is from dart:core

../../file/test/recording_test.dart _RecordingClass.futureProperty
../../file/test/recording_test.dart 78:12 main...

type '_ControllerStream' is not a subtype of type 'Stream' where
_ControllerStream is from dart:async
Stream is from dart:async
String is from dart:core

../../file/test/recording_test.dart _RecordingClass.streamMethod
../..file/test/recording_test.dart 96:36 main...

type 'LocalDirectory' is not a subtype of type 'FutureOr' of ':return_value' where
LocalDirectory is from package:file/src/backends/local/local_directory.dart
LocalDirectory is from package:file/src/backends/local/local_directory.dart
Directory is from dart:io
FutureOr is from dart:async
LocalDirectory is from package:file/src/backends/local/local_directory.dart

dart:async _Completer.completeError
package:file/src/forwarding/forwarding_file_system_entity.dart ForwardingFileSystemEntity.delete
===== asynchronous gap ===========================
dart:async _asyncThenWrapperHelper
package:file/src/forwarding/forwarding_file_system_entity.dart ForwardingFileSystemEntity.delete
../../file/test/common_tests.dart 732:28 runCommonTests....

and more

[keertip]

@matanlurey

[srawlins]

I'll take this one.