Fix await x++

tests/language/await_postfix_expr_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.
+
+import "dart:async";
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+
+post0(a) async {
+  return await a++;
+}
+
+post1(a) async {
+  return await a++ + await a++;
+}
+
+pref0(a) async {
+  return await ++a;
+}
+
+pref1(a) async {
+  return await ++a + await ++a;
+}
+
+sum(a) async {
+  var s = 0;
+  for (int i = 0; i < a.length; /* nothing */) {
+    s += a[await i++];
+  }
+  return s;
+}
+
+test() async {
+  Expect.equals(10, await post0(10));
+  Expect.equals(21, await post1(10));
+  Expect.equals(11, await pref0(10));
+  Expect.equals(23, await pref1(10));
+  Expect.equals(10, await sum([1, 2, 3, 4]));
+}
+
+main() {
+  asyncStart();
+  test().then((_) {
+    asyncEnd();
+  });
+}