Roll src/third_party/dart 79e6c74..9c649e6 (4 commits)

dart-lang/sdk@9c649e67df [cfe] Handle most accesses to extension members from within the extension
dart-lang/sdk@c7ea359627 Check for null tokens while walking backward from cursor
dart-lang/sdk@6408a9cc4d Tweaks for writeType() and summary2.
dart-lang/sdk@c179fa106e [dart2js] (New RTI) Handle extractTypeArguments.

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': '79e6c743371a319cfc58bcec76f4ffb3114ef9a9',
+  'dart_revision': '9c649e67dfac1825c388dc4867aa0beb486fe6c8',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py