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

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

Index: pkg/analyzer/lib/src/generated/workspace.dart
diff --git a/pkg/analyzer/lib/src/generated/workspace.dart b/pkg/analyzer/lib/src/generated/workspace.dart
index 503472b4eb4b5476b1e6a655792ba76158c9d6f1..eeff673a644c39c92f21d835c2a40b987e2f5915 100644
--- a/pkg/analyzer/lib/src/generated/workspace.dart
+++ b/pkg/analyzer/lib/src/generated/workspace.dart
@@ -25,12 +25,6 @@ abstract class Workspace {
   UriResolver get packageUriResolver;
 
   /**
-   * Return `true` if this workspace defines a single "project"
-   * and that "project" depends upon flutter,
-   */
-  bool get hasFlutterDependency => false;
-
-  /**
    * Create the [SourceFactory] for resolving Uris to [Source]s.
    * The [sdk] may be `null`.
    */