Declare compatibility with Dart 2 stable

[mit-mit]

The Dart SDK constraint needs to be updated to declare compatibility with Dart 2 stable:

https://github.com/google/platform.dart/blob/master/pubspec.yaml#L12

[mit-mit]

cc @tvolkert

[mkustermann]

The next flutter/engine -> flutter/flutter roll will receive a new 2.1.0-dev Dart SDK, which requires us to upgrade all 3rd party packages. Most of them have already newer versions published which are compatible with Dart 2, but package:platform is missing.

@tvolkert @goderbauer Would you mind publishing a new version with a sdk constraint including 2.1.0? (This is blocking flutter/flutter#20427)

[tvolkert]

Fixed with dart-archive/platform#18