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

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

 // Copyright (c) 2014, 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.source;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 486 matching lines @@
    *
    * @param localSourcePredicate the predicate to determine is [Source] is local
    */
   void set localSourcePredicate(LocalSourcePredicate localSourcePredicate);
 
   /// A table mapping package names to paths of directories containing
   /// the package (or [null] if there is no registered package URI resolver).
   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.
    */
   SourceFactory clone();
 
   /**
    * Return a source object representing the given absolute URI, or `null` if
    * the URI is not a valid URI or if it is not an absolute URI.
    *
    * @param absoluteUri the absolute URI to be resolved
@@ skipping 245 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]);
 
   /**
    * Return an absolute URI that represents the given [source], or `null` if a
    * valid URI cannot be computed.
    *
    * The computation should be based solely on [source.fullName].
    */
   Uri restoreAbsolute(Source source) => null;
 }