Removing dart:html APIs which were deprecated in M2.

tools/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.
 
 part of html;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME() => _$(CLASSNAME)FactoryProvider.createDocumentFragment();
 
   factory $CLASSNAME.html(String html) =>
       _$(CLASSNAME)FactoryProvider.createDocumentFragment_html(html);
 
   factory $CLASSNAME.svg(String svgContent) =>
       _$(CLASSNAME)FactoryProvider.createDocumentFragment_svg(svgContent);
 
-  @deprecated
-  List<Element> get elements => this.children;
-
-  // TODO: The type of value should be Collection<Element>. See http://b/5392897
-  @deprecated
-  void set elements(value) {
-    this.children = value;
-  }
-
 $if DART2JS
   // Native field is used only by Dart code so does not lead to instantiation
   // of native classes
   @Creates('Null')
 $endif
   List<Element> _children;
 
   List<Element> get children {
     if (_children == null) {
       _children = new FilteredElementList(this);
@@ skipping 40 matching lines @@
   void appendText(String text) {
     this.nodes.add(new Text(text));
   }
 
   void appendHtml(String text) {
     this.nodes.add(new DocumentFragment.html(text));
   }
 
 $!MEMBERS
 }