Roll src/third_party/dart 439caba..a9ea936 (4 commits) (#15886)

https://dart.googlesource.com/sdk.git/+log/439cabaec02f..a9ea9365ad8a

git log 439caba..a9ea936 --date=short --first-parent --format='%ad %ae %s'
2020-01-22 eernst@google.com Change null-aware `operator []`, `operator []=`, and function call to not have a period.
2020-01-22 eernst@google.com Migrate lib_2/typed_data to NNBD.
2020-01-22 johnniwinther@google.com [cfe] Use legacy erasure and nnbd top merge in computation of supertypes in ClassHierarchyBuilder
2020-01-22 johnniwinther@google.com [cfe] Test supertypes computed from ClassHierarchyBuilder

Created with:
  gclient setdep -r src/third_party/dart@a9ea9365ad8a

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/+/master/autoroll/README.md

Bug: None
Tbr: dart-vm-team@google.com

Author: skia-flutter-autoroll

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': '439cabaec02f5d9f219f3d7d33ec29cad167863c',
+  'dart_revision': 'a9ea9365ad8a3e3b59115bd889a55b6aa2c5a5fa',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py