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

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

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index 262bfe3b2d79c0b3b4398035afc2a321d4757b1c..65d663c56a542fc7fcd574ab39df18f5ee5d5c5c 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -504,6 +504,11 @@ abstract class SourceFactory {
   Map<String, List<Folder>> get packageMap;
 
   /**
+   * Return `true` if this collection of sources depends upon flutter,
+   */
+  bool get hasFlutterDependency => false;
+
+  /**
    * Return a source factory that will resolve URI's in the same way that this
    * source factory does.
    */