Make Stream abstract as it is in the actual SDK.

pkg/analyzer/test/generated/analysis_context_factory.dart

 // Copyright (c) 2016, 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.generated.analysis_context_factory;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 220 matching lines @@
         ElementFactory.classElement2("Completer", ["T"]);
     ConstructorElementImpl completerConstructor =
         ElementFactory.constructorElement2(completerElement, null);
     completerElement.constructors = <ConstructorElement>[completerConstructor];
     // StreamSubscription
     ClassElementImpl streamSubscriptionElement =
         ElementFactory.classElement2("StreamSubscription", ["T"]);
     // Stream
     ClassElementImpl streamElement =
         ElementFactory.classElement2("Stream", ["T"]);
+    streamElement.abstract = true;
     streamElement.constructors = <ConstructorElement>[
       ElementFactory.constructorElement2(streamElement, null)
     ];
     DartType returnType = streamSubscriptionElement.type
         .instantiate(streamElement.type.typeArguments);
     FunctionElementImpl listenOnData = ElementFactory.functionElement3(
         'onData',
         VoidTypeImpl.instance.element,
         <TypeDefiningElement>[streamElement.typeParameters[0]],
         null);
@@ skipping 320 matching lines @@
 
   void _addLibrary(LibraryMap map, String uri, bool isInternal, String path) {
     SdkLibraryImpl library = new SdkLibraryImpl(uri);
     if (isInternal) {
       library.category = "Internal";
     }
     library.path = path;
     map.setLibrary(uri, library);
   }
 }