Prevent app.stop/app.detach being sent twice
Prevent app.stop/app.detach being sent twice

Fixes https://github.com/Dart-Code/Dart-Code/issues/1848.
See https://github.com/flutter/flutter/issues/32651#issuecomment-505912091.

Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code