Turn off `strong_mode_down_cast_composite` warnings by default in Flutter projects.

pkg/analyzer/lib/src/context/source.dart

Index: pkg/analyzer/lib/src/context/source.dart
diff --git a/pkg/analyzer/lib/src/context/source.dart b/pkg/analyzer/lib/src/context/source.dart
index 9ea1d7dc8e3a63ae7ad3c94df862a56ed9b27e71..41487ec09ef81ee8b0b9751ae985c04a4c6f236c 100644
--- a/pkg/analyzer/lib/src/context/source.dart
+++ b/pkg/analyzer/lib/src/context/source.dart
@@ -102,6 +102,10 @@ class SourceFactoryImpl implements SourceFactory {
   }
 
   @override
+  // Alternately, we could check the pubspec for "sdk: flutter"
+  bool get hasFlutterDependency => packageMap.containsKey('flutter');
+
+  @override
   SourceFactory clone() {
     SourceFactory factory =
         new SourceFactory(resolvers, _packages, _resourceProvider);