Make Stream abstract as it is in the actual SDK.

pkg/analyzer/test/generated/non_error_resolver_test.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.generated.non_error_resolver_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 3357 matching lines @@
 }''');
     await computeAnalysisResult(source);
     assertNoErrors(source);
     verify([source]);
   }
 
   test_issue_24191() async {
     Source source = addSource('''
 import 'dart:async';
 
-class S extends Stream {}
+abstract class S extends Stream {}
 f(S s) async {
   await for (var v in s) {
     print(v);
   }
 }
 ''');
     await computeAnalysisResult(source);
     assertNoErrors(source);
     verify([source]);
   }
@@ skipping 2873 matching lines @@
   }
 
   Future<Null> _check_wrongNumberOfParametersForOperator1(String name) async {
     await _check_wrongNumberOfParametersForOperator(name, "a");
   }
 
   Future<CompilationUnit> _getResolvedLibraryUnit(Source source) async {
     return analysisContext.getResolvedCompilationUnit2(source, source);
   }
 }