Roll Dart SDK from f142aa958951 to 61d949d7ec1b (1 revision) (#25275)

DEPS

@@ -35,7 +35,7 @@ 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': 'f142aa958951f70333f51eab05a8d3950fabce14',
+  'dart_revision': '61d949d7ec1bccae40cdd25b59fa24b1b46ba714',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py