Revert "Roll src/third_party/dart 6ef9131..5829fc7 (3 commits) (flutter#16728)"

This reverts commit a150be2.

Author: NoamDev

DEPS

@@ -34,7 +34,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '5829fc7829d577a84e1296118bc3aa18deda670b',
+  'dart_revision': '6ef9131d82c4fbc8966f426bb865140ea3b7e860',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py