Roll src/third_party/dart c790eccdf4c4..7070347bbb7a (4 commits) (#15846)

https://dart.googlesource.com/sdk.git/+log/c790eccdf4c4..7070347bbb7a

git log c790eccdf4c4..7070347bbb7a --date=short --first-parent --format='%ad %ae %s'
2020-01-20 brianwilkerson@google.com Generate single quoted strings where possible
2020-01-20 brianwilkerson@google.com Enable prefer_single_quotes in analyzer_cli
2020-01-20 brianwilkerson@google.com Dartfix: add fixes, cleanup output, fix bug
2020-01-20 lrn@google.com Make the NNBD List.empty default to non-growable.

Created with:
  gclient setdep -r src/third_party/dart@7070347bbb7a

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': 'c790eccdf4c4c870d788ca5ff13580b8ea974949',
+  'dart_revision': '7070347bbb7aae91bcbb72982611c9806686f1d3',
 
   # 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: 98ac9923a77b1202e0b72add9fae6cbc
+Signature: 7dca0b6af1df19c27a8da0a1e748f003
 
 UNUSED LICENSES: