Roll Dart SDK from 62903873df30 to 249cb9c974e1 (6 revisions) (#18843)

https://dart.googlesource.com/sdk.git/+log/62903873df30..249cb9c974e1

2020-06-05 sra@google.com [benchmark] BigIntParsePrint lint fixes
2020-06-05 bkonyi@google.com [ VM Service / DDS ] Redirect web socket connection requests made to VM Service to DDS
2020-06-05 rmacnak@google.com [vm, gc] Prevent a program with sufficient idle time and dominated by direct old-space allocations from avoiding scavenges for too long.
2020-06-04 scheglov@google.com Implement allowedExperimentsJson for EmbedderSdk.
2020-06-04 rnystrom@google.com Migrate language_2/number to NNBD.
2020-06-04 scheglov@google.com Fix type cast exception in AddExplicitCast.

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/+doc/master/autoroll/README.md

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': '62903873df30775fefda0a8d99029a3c110bb299',
+  'dart_revision': '249cb9c974e13177fe5f76b390e2569ece79185f',
 
   # 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: 660854dea837219ed645658046349ce7
+Signature: a64a09a5666bcde41231353950203274
 
 UNUSED LICENSES: