Revert "Avoid warning when using `return;` from sync* and async* functions."

tests/compiler/dart2js/mock_libraries.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 for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 386 matching lines @@
   'DeferredLibrary': 'class DeferredLibrary {}',
   'Future':
       '''
       class Future<T> {
         Future.value([value]);
       }
       ''',
   'Stream': 'class Stream<T> {}',
   'Completer': 'class Completer<T> {}',
   'StreamIterator': 'class StreamIterator<T> {}',
-  '_SyncStarIterable': 'class _SyncStarIterable {}',
-  '_IterationMarker': 'class _IterationMarker {}',
-  '_AsyncStarStreamController': 'class _AsyncStarStreamController {}',
 };
 
 /// These members are only needed when async/await is used.
 const Map<String, String> ASYNC_AWAIT_LIBRARY = const <String, String>{
   '_wrapJsFunctionForAsync': '_wrapJsFunctionForAsync(f) {}',
   '_asyncHelper': '_asyncHelper(o, f, c) {}',
 };
 
 const String DEFAULT_MIRRORS_SOURCE = r'''
 import 'dart:_js_mirrors' as js;
@@ skipping 23 matching lines @@
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
   '_version': 'const _version = "0.0.1+1";',
 };