Roll dart to d948962. (flutter#12255)

* Roll dart to d948962.

Changes since last dart roll
```
d948962 Revert "[build] Speed up debug and simulator builds by running steps on the prebuilt VM."
35d1333 [vm] remove token position for ExceptionHandlers
5beca80 [analyzer] add isNullAware to IndexExpression
2bc48ad [analyzer] add isNullAware to propertyAccess
cfd1efc [vm/bytecode] Fix getting of target of dynamic invocation forwarder
68bcc00 [analyzer] implement isNullAware for method invocations
351acd1 [build] Fix application_snapshot.gni for uses outside of utils/xyz.
7badd80 Add a fix to replace an override with the extension name if accessing a static member
c72a858 Don't use DartType.instantiate() in TypeProviderImpl.
49de78b Convert tests so that they will continue to pass when extension methods are enabled by default
9d3de98 fix for REMOVE_UNUSED_LABEL
97925ec Delete test_runner/tool/co19.dart
6802251 Revert "[vm, service] Remove _collectAllGarbage."
892187e [dart2js] Ensure JSInvocationMirror.typeArguments is unmodifiable.
81c9fb8 Fix corelib_2/nsm_invocation_generic_test.
6a2a202 Auto-CC dart2js on changes to the js_runtime sdk.
29ce935 Migration: add a test case documenting a problem with "is" checks.
152fbbb Migration: add a test case documenting an operator== corner case.
83930fd Reformatted extension method tests.
4331e7e Fix bad tests setter_no_getter_test and syntax_test.
fc2f744 Deprecate 'Source.encoding'.
74cff6c [build] Speed up debug and simulator builds by running steps on the prebuilt VM.
59bcc4e [vm/kernel] record yield positions at streaming flow graph builder
c74e68e Add documentation for more diagnostics
59008b8 Skip parts with invalid URIs.
3b1d3de update parser to report out of order "?.." operator
420b292 [vm/compiler] More work on the IL serializer.
ebf1f01 [vm/compiler] Continued work on the IL deserializer.
bdf237f (origin/base) [cfe] Add a bit to Library to inticate if it's opted in for NNBD
a350f6e [gardening] Always use Dart binary from checked-in sdk when running gen_kernel in vm/dart tests
94b3a3f [cfe] Don't reuse type parameters of FunctionNode in its functionType
a9a9765 [gardening] Skip dart/run_appended_aot_snapshot_test on simarm_x64
8b26ae9 Change name of test configuration on abi test builder.
ce5556a [kernel] Load classes lazily
2bfe714 [kernel] Make class hierarchy member lists lazy
a622894 Update changelog to include the recent breaking change
60ca2b8 Revert "Enable visibleForTesting on unnamed constructors"
```

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': 'c74e68e50170e9d3965b7d649d22ceba939995ba',
+  'dart_revision': 'd9489622befb638c040975163cf9b8eba2ff057b',
 
   # 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: a01128ba9170fb0a6af4ae085427468e
+Signature: c1159f65a108cc6fff4fa6ad8ea69b83
 
 UNUSED LICENSES: