Fix Dart 2 issues in firebase plugins (#403)
Fix Dart 2 issues in firebase plugins (#403)

Most of the plugins now pass all the tests.

Fixes flutter/flutter#15027

fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins