Ensure Dart is present in update_flutter_engine (#64)
Ensure Dart is present in update_flutter_engine (#64)

A clean Flutter doesn't contain Dart; check whether Dart is present
before trying to use it, and if not run 'flutter precache'.

Fixes issue #63
google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding