Roll Dart SDK from 39dc634 to 4db0f55 (1 revision) (flutter-team-archive#24431)

Author: skia-flutter-autoroll

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': '39dc634393968ee146c5b3ffe7d5d0680c65e7ab',
+  'dart_revision': '4db0f5599c85e5bf060e6a851997789e09a63718',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py