Add support for sync* and yield

tests/language/async_await_syntax_test.dart

 // Copyright (c) 2015, 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.
 
 // Test async/await syntax.
 
 import 'dart:async' show Stream;
 
 var yield = 0;
 var await = 0;
 get st => new Stream.fromIterable([]);
 
 a01a() async => null;                        /// a01a: ok
 a01b() async* => null;                       /// a01b: compile-time error
 a01c() sync* => null;                        /// a01c: compile-time error
 a02a() async {}                              /// a02a: ok
 a03a() async* {}                             /// a03a: ok
-a03b() async * {}                            /// a03b: compile-time error
+a03b() async * {}                            /// a03b: ok
 a04a() sync* {}                              /// a04a: ok
 a04b() sync {}                               /// a04b: compile-time error
-a04c() sync * {}                             /// a04c: compile-time error
+a04c() sync * {}                             /// a04c: ok
 a05a() async { await 0; }                    /// a05a: ok
 a05b() async {                               /// a05b: ok
   await(a) {};                               /// a05b: continued
   await(0);                                  /// a05b: continued
 }                                            /// a05b: continued
 a05c() {                                     /// a05c: ok
   await(a) {};                               /// a05c: continued
   await(0);                                  /// a05c: continued
 }                                            /// a05c: continued
 a05d() async {                               /// a05d: compile-time error
@@ skipping 235 matching lines @@
   a = c.async;  /// b14c: continued
   a = c.async;  /// b14d: continued
   c.sync();     /// b15a: continued
   c.sync();     /// b15b: continued
   c.async();    /// b15c: continued
   c.async();    /// b15d: continued
 
   method1();
   method2();
 }