Use declared return type for inference of return and yield statements.

pkg/analyzer/test/src/context/mock_sdk.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.test.src.context.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
@@ skipping 54 matching lines @@
 ''',
     const <String, String>{
       '$sdkRoot/lib/async/stream.dart': r'''
 part of dart.async;
 abstract class Stream<T> {
   Future<T> get first;
   StreamSubscription<T> listen(void onData(T event),
                                { Function onError,
                                  void onDone(),
                                  bool cancelOnError});
+  Stream();
+  factory Stream.fromIterable(Iterable<T> data);
 }
 
 abstract class StreamSubscription<T> {
   Future cancel();
   void onData(void handleData(T data));
   void onError(Function handleError);
   void onDone(void handleDone());
   void pause([Future resumeSignal]);
   void resume();
   bool get isPaused;
@@ skipping 542 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }