Allow await in assert (fix issue 22728).

tests/language/regress_22728_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 'package:expect/expect.dart';
+
+bool assertsChecked() {
+  bool checked = false;
+  try {
+    assert(false);
+  } on AssertionError catch (error) {
+    checked = true;
+  }
+  return checked;
+}
+
+main() async {
+  bool fault = false;
+  try {
+    assert(await false);
+  } on AssertionError catch (error) {
+    fault = true;
+  }
+  Expect.equals(assertsChecked(), fault);
+}