Roll src/third_party/dart 83eeab1e63d9..6c99171686be (5 commits) (#14492)

https://dart.googlesource.com/sdk.git/+log/83eeab1e63d9..6c99171686be

git log 83eeab1e63d9..6c99171686be --date=short --first-parent --format='%ad %ae %s'
2019-12-14 scheglov@google.com Report CompileTimeErrorCode.EXPORT_LEGACY_SYMBOL
2019-12-14 asiva@google.com [SDK/BUILD] Pass --enable-experiment=non-nullable to the gen_kernel when it is used to compile the sdk tools.
2019-12-14 srawlins@google.com Highlight line that is targeted; fix some linking
2019-12-14 srujzs@google.com Change Future lists to use dynamic
2019-12-14 srawlins@google.com Fix pedantic 0.9 lints around collection literals and spreads

Created with:
  gclient setdep -r src/third_party/dart@6c99171686be

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': '83eeab1e63d9128879992916ba1b58151264c132',
+  'dart_revision': '6c99171686be288f1954ddbaa4035b99b3196197',
 
   # 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

-Signature: 959d54ea31b46ebde73248d034a42d09
+Signature: 03050e85894e594fc1f9e8b214b918da
 
 UNUSED LICENSES: