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

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

 // Copyright (c) 2015, 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.source;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/exception/exception.dart';
@@ skipping 84 matching lines @@
       return packageMap;
     }
 
     // Default to the PackageMapUriResolver.
     PackageMapUriResolver resolver = resolvers
         .firstWhere((r) => r is PackageMapUriResolver, orElse: () => null);
     return resolver?.packageMap;
   }
 
   @override
+  // Alternately, we could check the pubspec for "sdk: flutter"
+  bool get hasFlutterDependency => packageMap.containsKey('flutter');
+
+  @override
   SourceFactory clone() {
     SourceFactory factory =
         new SourceFactory(resolvers, _packages, _resourceProvider);
     factory.localSourcePredicate = _localSourcePredicate;
     return factory;
   }
 
   @override
   Source forUri(String absoluteUri) {
     try {
@@ skipping 145 matching lines @@
       for (UriResolver resolver in resolvers) {
         Source result = resolver.resolveAbsolute(containedUri, actualUri);
         if (result != null) {
           return result;
         }
       }
       return null;
     });
   }
 }