Roll src/third_party/dart 6bf1f8e280..63120303a7 (4 commits)

dart-lang/sdk@63120303a7 [vm] Guard against double finalization from annotations on fields.
dart-lang/sdk@f664a80850 [vm/socket] check nulls of rawsocket before operation
dart-lang/sdk@ff03f01759 null_closures dartfix
dart-lang/sdk@671bcffa3f [js_runtime] Run dartfmt

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': '6bf1f8e280f29eee422c3c45a288f4100448f0cc',
+  'dart_revision': '63120303a7496cb36967b69937cadd2c39e82386',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py

ci/licenses_golden/licenses_third_party

-Signature: 52bdc7348e1d160fcea1dc48ae16b3c6
+Signature: f423bdfc4a19fe3371de483d97a7f4c0
 
 UNUSED LICENSES: