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

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.workspace;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * Abstract superclass of classes that provide information about the workspace
  * in which analysis is being performed.
  */
 abstract class Workspace {
 
   /**
    * Return a map of package sources.
    */
   Map<String, List<Folder>> get packageMap;
 
   /**
    * The [UriResolver] that can resolve `package` URIs.
    */
   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`.
    */
   SourceFactory createSourceFactory(DartSdk sdk);
 }