Roll src/third_party/dart 5b7c981086..5e39817ec7 (2 commits)

dart-lang/sdk@5e39817ec7 [vm] Don't use canonical names in the protobuf treeshaker
dart-lang/sdk@d7c75edd57 [CFE] Use 'SyntheticBeginToken' for BeginToken that origins from a synthetic node

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': '5b7c981086141d367077c9b9a8efdb7e9fc5d784',
+  'dart_revision': '5e39817ec7ab7f56f381c244d105c7e40913a3e0',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py