Fix a type inference corner case: return expression where void expected.

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 252 matching lines @@
   E get first;
 
   Iterable/*<R>*/ map/*<R>*/(/*=R*/ f(E e));
 
   /*=R*/ fold/*<R>*/(/*=R*/ initialValue,
       /*=R*/ combine(/*=R*/ previousValue, E element));
 
   Iterable/*<T>*/ expand/*<T>*/(Iterable/*<T>*/ f(E element));
 
   Iterable<E> where(bool test(E element));
+  
+  void forEach(void f(E element));
 
   List<E> toList();
 }
 
 class List<E> implements Iterable<E> {
   List();
   void add(E value) {}
   void addAll(Iterable<E> iterable) {}
   E operator [](int index) => null;
   void operator []=(int index, E value) {}
@@ skipping 342 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }