Roll src/third_party/dart 171059d..aa67099 (11 commits) (#14567)

https://dart.googlesource.com/sdk.git/+log/171059d27689..aa6709974dea

git log 171059d..aa67099 --date=short --first-parent --format='%ad %ae %s'
2019-12-19 kallentu@google.com [cfe] Fix inference for variant parameters leaking UnknownTypes.
2019-12-19 brianwilkerson@google.com Add support to dartfix for prefer_single_quotes
2019-12-19 brianwilkerson@google.com Enable prefer_is_empty and prefer_is_not_empty in analyzer_plugin and analyzer_cli
2019-12-19 brianwilkerson@google.com Remove unnecessary uses of const in analysis_server_client
2019-12-19 nshahan@google.com [dartdevc] Add an nnbd runtime mode weak/strong option to ddb script
2019-12-18 sigmund@google.com Add ddc nnbd legacy tests
2019-12-18 jsimmons@google.com [observatory] Use month numbers ranging from 1-12 in the timeline default filename
2019-12-18 nbosch@google.com Remove int return from main and drop `new`
2019-12-18 sjindel@google.com [vm] Remove redundant type-tests from min/max.
2019-12-18 fizaaluthra@google.com [dartfuzz] Put gen_api_table.dart constants in alphabetical order
2019-12-18 scheglov@google.com Implement NNBD_TOP_MERGE.

Created with:
  gclient setdep -r src/third_party/dart@aa6709974dea

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': '171059d2768967b8b17e8ce33f09ce413c63b69a',
+  'dart_revision': 'aa6709974dea00680588be951f05d0f36fb8576b',
 
   # 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: f1b01cca605a48a58c06e37f300d0e79
+Signature: 3bce57e8ca72b8cadaab0bccec6fa28d
 
 UNUSED LICENSES: