Fix conditionally parsing await as keyword.

tests/language/await_backwards_compatibility_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
+
+import 'dart:async';
+
+import 'package:expect/expect.dart';
+
+get await => 4;
+
+// For functions that are declared with the async modifier we treat await as
+// keyword.
+
+test0() async {
+  var x = await 7;
+  Expect.equals(7, x);
+  var await = 1;  /// await1: compile-time error
+}
+
+test1() async {
+  var x = await 9;
+  Expect.equals(9, x);
+  var y = await;  /// await2: compile-time error
+}
+
+// For functions that are not declared with the async modifier we allow await to
+// be used as an identifier.
+
+test2() {
+  var y = await;
+  Expect.equals(4, y);
+  var x = await 1;  /// await3: compile-time error
+}
+
+test3() {
+  var await = 3;
+  Expect.equals(3, await);
+  var x = await 1;  /// await4: compile-time error
+}
+
+main() {
+  test0();
+  test1();
+  test2();
+  test3();
+}