Roll Dart SDK from c4214e6daaac to 4a6764bf28c2 (4 revisions) (flutter#23770)

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': 'c4214e6daaaca2d804ee2909d49fdd988629e796',
+  'dart_revision': '4a6764bf28c2ef4cd148177135b32af954b14d10',
 
   # 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: 06a8e3c89c2790f444a431cdbe069209
+Signature: 546f2e94919d9de79e2d2b0de2664a24
 
 UNUSED LICENSES:
 

tools/const_finder/.dart_tool/package_config.json

@@ -11,7 +11,7 @@
       "name": "kernel",
       "rootUri": "../../../../third_party/dart/pkg/kernel",
       "packageUri": "lib",
-      "languageVersion": "2.6"
+      "languageVersion": "2.12"
     },
     {
       "name": "meta",