Roll Dart SDK from 6d06476 to a9e67d8 (3 revisions)

https://dart.googlesource.com/sdk.git/+log/6d06476bae6b..a9e67d81941a

2020-07-11 scheglov@google.com Prepare to publish analyzer_plugin 0.3.0
2020-07-11 scheglov@google.com Add Scope and LibraryElement.scope to API.
2020-07-11 mfairhurst@google.com [analysis_server] Remove added space.

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
Please CC dart-vm-team@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

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': '6d06476bae6b0d1c40f6b3c9e0ef874ec3b631b7',
+  'dart_revision': 'a9e67d81941a085ba6b8d0a2fe959c47e5a8b3a2',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py