Make LinkedHashMap also have a factory constructor and be customizable

pkg/mdv/lib/src/template_iterator.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.
 
 part of mdv;
 
 // This code is a port of Model-Driven-Views:
 // https://github.com/polymer-project/mdv
 // The code mostly comes from src/template_element.js
 
@@ skipping 326 matching lines @@
     var delegate = template.bindingDelegate;
 
     if (template.parentNode == null || template.document.window == null) {
       close();
       // TODO(jmesserly): MDV calls templateIteratorTable.delete(this) here,
       // but I think that's a no-op because only nodes are used as keys.
       // See https://github.com/Polymer/mdv/pull/114.
       return;
     }
 
-    // TODO(jmesserly): IdentityMap matches JS semantics, but it's O(N) right
-    // now. See http://dartbug.com/4161.
-    var instanceCache = new IdentityMap();
+    var instanceCache = new HashMap(equals: identical);
     var removeDelta = 0;
     for (var splice in splices) {
       for (int i = 0; i < splice.removedCount; i++) {
         var instanceNodes = extractInstanceAt(splice.index + removeDelta);
         if (instanceNodes.length == 0) continue;
         var model = _mdv(instanceNodes.first)._templateInstance.model;
         instanceCache[model] = instanceNodes;
       }
 
       removeDelta -= splice.addedCount;
@@ skipping 47 matching lines @@
         nodeExt._templateIterator = null;
       }
     }
 
     _nodeOrCustom(node).unbindAll();
     for (var c = node.firstChild; c != null; c = c.nextNode) {
       _unbindAllRecursively(c);
     }
   }
 }