Remove old TODO and comments
Remove old TODO and comments

TODO is no longer valid: dart2js avoid type casts in JS when compiling
implicit downcasts.

See also related issue #712

dart-lang/protobuf,dart-lang/dart-protobuf,google/protobuf.dart,google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/dart-protobuf,google/protobuf.dart,dart-lang/protobuf,dart-lang/protobuf