Make Stream abstract as it is in the actual SDK.

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 48 matching lines @@
   factory Completer.sync() => new _SyncCompleter<T>();
   Future<T> get future;
   void complete([value]);
   void completeError(Object error, [StackTrace stackTrace]);
   bool get isCompleted;
 }
 ''',
     const <String, String>{
       '$sdkRoot/lib/async/stream.dart': r'''
 part of dart.async;
-class Stream<T> {
+abstract class Stream<T> {
   Future<T> get first;
 }
 abstract class StreamTransformer<S, T> {}
 '''
     });
 
 const _MockSdkLibrary _LIB_COLLECTION = const _MockSdkLibrary(
     'dart:collection',
     '$sdkRoot/lib/collection/collection.dart',
     '''
@@ skipping 455 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }