Revert "Roll src/third_party/dart 6aa7407..e84bea2 (6 commits) (flutter#15934)"

This reverts commit a1db410.

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': 'e84bea25df23ea5f149fc808a6423c7500b9cace',
+  'dart_revision': '6aa7407d63092696752c68dd0426c8ce99e5c6a8',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py