Roll src/third_party/dart 306f8e04bb..fecc4c8f2d (4 commits)

dart-lang/sdk@fecc4c8f2d Breaking changes for analyzer version 0.38.
dart-lang/sdk@8b8c832287 Disable summary1 (pre)linking tests when useSummary2.
dart-lang/sdk@5f824e572d Return extensions from search.topLevelDeclarations
dart-lang/sdk@100d500b45 Update test expectations to match analyzer behavior

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': '306f8e04bbbf00eaa8eb7c27f1f92a3963a823e0',
+  'dart_revision': 'fecc4c8f2de2b2cfb7d20a0c780f20486372db01',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py