Bump Dart SDK to `1.21.0-dev.9.0`. (#6990)

* Fixes Analysis Server regression (https://github.com/dart-lang/sdk/issues/27856).
* Adds `front_end` dependency override (required by analyzer).

bin/internal/dart-sdk.version

-1.21.0-dev.6.0
+1.21.0-dev.9.0

packages/flutter_tools/pubspec.yaml

@@ -36,6 +36,7 @@ dependencies:
 
   # Version from the vended Dart SDK as defined in `dependency_overrides`.
   analyzer: any
+  front_end: any
 
 dev_dependencies:
   mockito: ^1.0.0
@@ -47,3 +48,5 @@ dartdoc:
 dependency_overrides:
   analyzer:
     path: ../../bin/cache/dart-sdk/lib/analyzer
+  front_end:
+    path: ../../bin/cache/dart-sdk/lib/front_end