Make LinkedHashMap also have a factory constructor and be customizable

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 LinkedHashMap<String,String> {
+class Crash extends HashSet<String> {
   Crash(): super();
 }
 
 void main() {
   Crash map = new Crash();
-  Expect.isTrue(map is LinkedHashMap);
+  Expect.isTrue(map is HashSet);
 }