Fix crash reported in http://code.google.com/p/dart/issues/detail?id=9949.

tests/language/issue9949_test.dart

+// Copyright (c) 2013, 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.
+
+// Regression test for dart2js that used to crash in the presence of a
+// super constructor declared external.
+
+import "package:expect/expect.dart";
+import 'dart:collection';
+
+class Crash extends HashMap<String,String> {
+  Crash(): super();
+}
+
+void main() {
+  Crash map = new Crash();
+  Expect.isTrue(map is HashMap);
+}