Roll Dart SDK from 6b630c3 to f4047fc (15 revisions)

https://dart.googlesource.com/sdk.git/+log/6b630c3719bd..f4047fc96f6d

2020-06-24 regis@google.com Roll benchmarks-internal to cb9ae13486f5dc0ec8d80694dc6b8a50dc4ac7d1
2020-06-23 srujzs@google.com [dart:html] Modify scripts to always use NNBD
2020-06-23 rnystrom@google.com Correctly run legacy tests.
2020-06-23 alexmarkov@google.com [vm/nnbd] Test for type casts with null-safety auto-detection
2020-06-23 scheglov@google.com Clean up matching type parameters in FunctionExpressionResolver.
2020-06-23 rmacnak@google.com [vm] Use unsigned arithmetic when computing CID masks.
2020-06-23 scheglov@google.com Fix UNUSED_LOCAL_VARIABLE in UnnecessaryCastTest.
2020-06-23 scheglov@google.com Check for PackageBuildPackageUriResolver in SourceFactory.packageMap
2020-06-23 devoncarew@google.com [dartev] address a few analysis issues in pkg/dartev
2020-06-23 alexmarkov@google.com [vm/aot/tfa] Re-implement protobuf-aware tree shaker inside TFA
2020-06-23 jwren@google.com This new tool resolves and evaluates field declarations where the type isn't specified, but is known when the code is resolved, i.e. cases of var i = 1;.
2020-06-23 nbosch@google.com Remove a debug print statement
2020-06-23 scheglov@google.com Check for null in VariableDeclaration context type.
2020-06-23 brianwilkerson@google.com Generate tables of keyword relevance
2020-06-23 srawlins@google.com Migrator: add --preview-hostname option

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

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': '6b630c3719bd560399a5fb797970813630d510d8',
+  'dart_revision': 'f4047fc96f6db96dd1df7e8405982127dcf5477f',
 
   # 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: 6af1fc7613bab1cfddbc06661248abf4
+Signature: 7706727214570bd140961ec4355c750d
 
 UNUSED LICENSES: