Make LinkedHashMap also have a factory constructor and be customizable

tests/language/issue10561_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 miscompile classes
 // extending HashMap, because HashMap is patched.
 
 import "package:expect/expect.dart";
 
 import 'dart:collection';
 
-class Foo extends LinkedHashMap {
+class Foo extends HashSet {
 }
 
 main() {
   Expect.equals(0, new Foo().length);
 }