Roll src/third_party/dart b173229baa..76c99bcd01 (5 commits)

dart-lang/sdk@76c99bcd01 Add tests for internal name conflicts in extension methods
dart-lang/sdk@6c46a93361 Check whether completion has been aborted before making model request or blocking on response
dart-lang/sdk@9776e3699f Tests and fixes for searching extensions and extension members.
dart-lang/sdk@b6455ab53a Expand the version range for dart_internal to allow 2.5.x versions.
dart-lang/sdk@37a483ef78 Set incorrectly referenced static element, and test CHANGE_TO_STATIC_ACCESS.

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': 'b173229baa47de19e9f4dfb2c4908c90951cb951',
+  'dart_revision': '76c99bcd01c2679363adf385ea60dec8521fce35',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py