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

pkg/analyzer/lib/src/dart/analysis/library_analyzer.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.
 
 import 'package:analyzer/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/error/listener.dart';
@@ skipping 231 matching lines @@
     RecordingErrorListener errorListener = _getErrorListener(file);
 
     if (_analysisOptions.strongMode) {
       AnalysisOptionsImpl options = _analysisOptions as AnalysisOptionsImpl;
       CodeChecker checker = new CodeChecker(
           _typeProvider,
           new StrongTypeSystemImpl(_typeProvider,
               implicitCasts: options.implicitCasts,
               nonnullableTypes: options.nonnullableTypes),
           errorListener,
-          options);
+          options,
+          _sourceFactory.hasFlutterDependency);
       checker.visitCompilationUnit(unit);
     }
 
     ErrorReporter errorReporter = _getErrorReporter(file);
 
     //
     // Validate the directives.
     //
     _validateUriBasedDirectives(file, unit);
 
@@ skipping 496 matching lines @@
 }
 
 /**
  * Either the name or the source associated with a part-of directive.
  */
 class _NameOrSource {
   final String name;
   final Source source;
   _NameOrSource(this.name, this.source);
 }