Roll Dart to version 7f9a9fe (flutter#6366)

* Roll Dart to version 7f9a9fe

This contains the following commits:

7f9a9fe [vm, service] Rename 'invoke' parameter 'receiverId' to 'targetId'.
fa3d8fa Don't check for the file presence in transitive sets when we are just creating it.
6f3de54 [ VM / Hot Reload ] Fixed tests which were crashing periodically. Fixes issue flutter#34599
e811b29 [vm, gc] Restore growth policy when concurrent marking is not enabled.
0518b33 Address comments
7c46a28 Update convert class to mixin assist to remove "abstract"
06744c6 Pull reporting for unimplemented inherited members, and declared abstract members from ErrorVerifier.
45c7e43 Update dartfix to apply changes to file rather than displaying them
9ad59ba [dartdevc] fix flutter#34358, int to double conversion for literals
edddc0b Generate source changes for converting class to mixin
2742a5d [Gardening] Update test status
d9de98e [VM] Delete some dead code in gen_snapshot.cc
51b8fc7 AwaitExpression.precedence => 14
2390b36 Create a package for shared diagnostics
629d76e Simple dartfix that suggests class to be converted to mixins
8a2cfd8 Implement InheritanceManager2.getMember() and use it in some places.
71eaf32 Fix broken 'fasta testing sdk'.
d79f749 Test that type parameters of typedefs are properly bounds checked.
b36f4ff Allow multiple analyzer codes
82b3501 Fix supermixin handling in dart2js
c9b05d3 Allow super calls in mixin declarations
225b8d5 Specialize Object properties in static type computation
4a58112 Move kernel_impact.dart from ssa/ to kernel/
aaf3b70 [release] Prepare changelog for 2.1.0-dev.6.0
e9c9493 Remove unused parts of J/K*Env classes
4659cb9 Fully split JsToElementMap and KernelToElementMap
a617700 Removes code that has become redundant since 98a9adf.
7c54fab Adjust status files.
76470ac [VM] use coresnapshot when initializing an Isolate

* Address review comments.

DEPS

@@ -31,7 +31,7 @@ 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': '808ed6238b9262660e31ea826f7aea6cfa3a3493',
+  'dart_revision': '7f9a9feae181573cca609fd1b8760d7992176699',
 
   # 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: 298a107bb55f276ac38852045377a801
+Signature: 092b3d880fec4ee1110a9c0630e6da6a
 
 UNUSED LICENSES:
 

runtime/dart_isolate.cc

@@ -636,16 +636,6 @@ DartIsolate::CreateDartVMAndEmbedderObjectPair(
             (*raw_embedder_isolate)->child_isolate_preparer_));
   }
 
-  // TODO(rmacnak): This flag setting business preserves a side effect of using
-  // Dart_CreateIsolateFromKernel. It should be removed when some of the
-  // internal logic in reload no longer uses this flag.
-  Dart_IsolateFlags nonnull_flags;
-  if (flags == nullptr) {
-    Dart_IsolateFlagsInitialize(&nonnull_flags);
-    flags = &nonnull_flags;
-  }
-  flags->use_dart_frontend = true;
-
   // Create the Dart VM isolate and give it the embedder object as the baton.
   Dart_Isolate isolate = Dart_CreateIsolate(
       advisory_script_uri,         //