Implementing polyfills for insertAdjacent* to get them working on FF.

lib/dom/templates/html/impl/impl_DocumentFragment.darttemplate

 // Copyright (c) 2011, 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.
 
 class FilteredElementList implements ElementList {
   final Node _node;
   final NodeList _childNodes;
 
   FilteredElementList(Node node): _childNodes = node.nodes, _node = node;
 
@@ skipping 179 matching lines @@
     => this._insertAdjacentNode(where, element);
 
   void insertAdjacentText(String where, String text) {
     this._insertAdjacentNode(where, new Text(text));
   }
 
   void insertAdjacentHTML(String where, String text) {
     this._insertAdjacentNode(where, new DocumentFragment.html(text));
   }
 
+  void addText(String text) {
+    this.insertAdjacentText('beforeend', text);
+  }
+
+  void addHTML(String text) {
+    this.insertAdjacentHTML('beforeend', text);
+  }
+
   Future<ElementRect> get rect() {
     return _createMeasurementFuture(() => const EmptyElementRect(),
                                     new Completer<ElementRect>());
   }
 
   // If we can come up with a semi-reasonable default value for an Element
   // getter, we'll use it. In general, these return the same values as an
   // element that has no parent.
   String get contentEditable() => "false";
   bool get isContentEditable() => false;
@@ skipping 127 matching lines @@
       "WebKit drop zone can't be set for document fragments.");
   }
 
   void set webkitRegionOverflow(String value) {
     throw new UnsupportedOperationException(
       "WebKit region overflow can't be set for document fragments.");
   }
 
 $!MEMBERS
 }