Roll Dart SDK from 21b7f86 to f2a7851 (1 revision) (#24030)

https://dart.googlesource.com/sdk.git/+log/21b7f864e3b9..f2a7851b676d

2021-01-28 [email protected] Version 2.12.0-273.0.dev

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 [email protected],[email protected] 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/+doc/master/autoroll/README.md

Author: skia-flutter-autoroll

DEPS

@@ -35,7 +35,7 @@ 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': '21b7f864e3b9ff141639c33044d796aad259acb8',
+  'dart_revision': 'f2a7851b676d2487d1656e141a7e07a28c1db06a',
 
   # 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: acfa6a848fbdc00eb19e009ee5b1020a
+Signature: 3e93cff56fc7cd7df92bd669e72923eb
 
 UNUSED LICENSES: