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

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

 // Copyright (c) 2016, 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.context.context_builder;
 
 import 'dart:collection';
 import 'dart:core';
 
 import 'package:analyzer/context/declared_variables.dart';
@@ skipping 370 matching lines @@
     if (optionsFile != null) {
       try {
         optionMap = optionsProvider.getOptionsFromFile(optionsFile);
       } catch (_) {
         // Ignore exceptions thrown while trying to load the options file.
       }
     } else {
       // Search for the default analysis options
       Source source;
       // TODO(danrubel) determine if bazel or gn project depends upon flutter
-      if (workspace.hasFlutterDependency) {
+      if (sourceFactory.hasFlutterDependency) {
         source =
             sourceFactory.forUri('package:flutter/analysis_options_user.yaml');
       }
       if (source == null || !source.exists()) {
         source =
             sourceFactory.forUri('package:dart.analysis_options/default.yaml');
       }
       if (source.exists()) {
         try {
           optionMap = optionsProvider.getOptionsFromSource(source);
@@ skipping 321 matching lines @@
 
   final ContextBuilder _builder;
 
   Map<String, List<Folder>> _packageMap;
 
   Packages _packages;
 
   _BasicWorkspace._(this.provider, this.root, this._builder);
 
   @override
-  // Alternately, we could check the pubspec for "sdk: flutter"
-  bool get hasFlutterDependency => packageMap.containsKey('flutter');
-
-  @override
   Map<String, List<Folder>> get packageMap {
     _packageMap ??= _builder.convertPackagesToMap(packages);
     return _packageMap;
   }
 
   Packages get packages {
     _packages ??= _builder.createPackageMap(root);
     return _packages;
   }
 
@@ skipping 13 matching lines @@
   }
 
   /**
    * Find the basic workspace that contains the given [path].
    */
   static _BasicWorkspace find(
       ResourceProvider resourceProvider, String path, ContextBuilder builder) {
     return new _BasicWorkspace._(resourceProvider, path, builder);
   }
 }