Fix parsing of awaitable expressions with nested functions

tests/language/await_regression_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.
+//
+// VMOptions=--enable_async
+//
+// Regression test for issue 21536.
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+
+later(vodka) => new Future.value(vodka);
+
+manana(tequila) async => tequila;
+
+main() async {
+  var a = await later('Asterix').then((tonic) {
+    return later(tonic);
+  });
+  var o = await manana('Obelix').then(manana);
+  Expect.equals("$a and $o", "Asterix and Obelix");
+}