Revert "Roll src/third_party/dart b3457c8..32ce9c7 (7 commits) (flutter#15504)"

This reverts commit a3d7211.

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': '32ce9c7872e241b338502c56e78109fa079ff6c5',
+  'dart_revision': 'b3457c8cc2c1d8451d61fa8c6ab4a3d7529f7d76',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py