Roll src/third_party/dart 582727a..6aa7407 (4 commits) (flutter#15923)

https://dart.googlesource.com/sdk.git/+log/582727a4ec3b..6aa7407d6309

git log 582727a..6aa7407 --date=short --first-parent --format='%ad %ae %s'
2020-01-23 jensj@google.com [CFE] Proof of concept of using vm service to find leaks
2020-01-23 jensj@google.com [CFE] Add interactive mode to spell checker
2020-01-23 scheglov@google.com Update NNBD_TOP_MERGE for pending updates to the spec.
2020-01-23 asiva@google.com [VM/libraries] Fix more analyzer errors in patch files.

Created with:
  gclient setdep -r src/third_party/dart@6aa7407d6309

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': '582727a4ec3b4ec3ad145ff421499e54d683763b',
+  'dart_revision': '6aa7407d63092696752c68dd0426c8ce99e5c6a8',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py

ci/licenses_golden/licenses_third_party

@@ -1,4 +1,4 @@
-Signature: 37497488fec4c1e2f63b9cfffdd993cc
+Signature: 88259be2fef305304772d72997b86a96
 
 UNUSED LICENSES: