Roll src/third_party/dart 141fcfa..4681868 (9 commits) (flutter#14590)

https://dart.googlesource.com/sdk.git/+log/141fcfa61092..4681868aaf77

git log 141fcfa..4681868 --date=short --first-parent --format='%ad %ae %s'
2019-12-20 mfairhurst@google.com [nnbd_migration] Fix TypeParameterType asInstanceOf where valid.
2019-12-20 fizaaluthra@google.com [dartfuzz] Disable stopwatch
2019-12-20 brianwilkerson@google.com Fix an NPE when computing a proximateTargetForNode when the node is null
2019-12-20 scheglov@google.com Use DartType.getDisplayString() in as many places as possible.
2019-12-20 scheglov@google.com Remove SdkExtUriResolver.
2019-12-19 sigmund@google.com Cleanup sourcemap test
2019-12-19 nshahan@google.com Add more test suites to DDC NNBD strong/weak configs
2019-12-19 scheglov@google.com Issue 39773. Don't report MISSING_DEFAULT_VALUE_FOR_PARAMETER when no default value is allowed.
2019-12-19 sigmund@google.com Support building under git-worktree

Created with:
  gclient setdep -r src/third_party/dart@4681868aaf77

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

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': '141fcfa61092f89654056a34db7192dea8d9c9b3',
+  'dart_revision': '4681868aaf77a163f4778adfc599b7d6c407250f',
 
   # 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: b207ff878a9906eeb1d7fa1797f0cc31
+Signature: db05fbe326df1fe95126093dcdfb65d8
 
 UNUSED LICENSES: