Flutter Dart2 Support

[development-s365]

Using this package in a flutter test project gives following error:

E/flutter (22587): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter (22587): type 'Future<dynamic>' is not a subtype of type 'FutureOr<ConnectionImpl>' where
E/flutter (22587):   Future is from dart:async
E/flutter (22587):   FutureOr is from dart:async
E/flutter (22587):   ConnectionImpl is from package:postgresql2/src/postgresql_impl/postgresql_impl.dart

Switching flutter's SDK to disable dart 2 mode solves this issue but isn't really the solution long term.

[tomyeh]

Dart 2 supported: d4ab10a