Dart: Ignore .flutter-plugins (#3398)

Generated files that appear when using 'google_fonts' or other Dart
packages designed as Flutter plugins.

Dart.gitignore

@@ -19,3 +19,6 @@ doc/api/
 *.js_
 *.js.deps
 *.js.map
+
+.flutter-plugins
+.flutter-plugins-dependencies