Support enums in dart2dart.

tests/language/enum_index_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.
+
+// SharedOptions=--enable-enum
+
+// Test index access for enums.
+
+library enum_index_test;
+
+import 'package:expect/expect.dart';
+
+enum Enum {
+  A,
+  B,
+}
+
+class Class {
+  var index;
+}
+
+main() {
+  test(null, new Class());
+  test(0, Enum.A);
+  test(1, Enum.B);
+}
+
+test(expected, object) {
+  Expect.equals(expected, object.index);
+}
+
+