Add some sweet XML classes.

client/html/src/DocumentFragment.dart

 // 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.
 
 interface DocumentFragment extends Element default DocumentFragmentWrappingImplementation {
  
   DocumentFragment();
 
   DocumentFragment.html(String html);
 
+  DocumentFragment.xml(String xml);

[Jacob, 2012/02/22 21:55:00]

should we include this method? we don't really want to encourage people to
heavily use XML.

[nweiz, 2012/02/23 00:35:06]

Because we allow all Element sub-interfaces to function as Elements, it's very
useful to have something like DocumentFragment that contains multiple elements
without a root. It makes some things much, much easier; see e.g. the
implementation of XMLElementWrappingImplementation#_insertAdjacentNode. We may
not want to encourage XML, but we don't want to make it painful either.

+
+  DocumentFragment.svg(String svg);
+
   DocumentFragment clone(bool deep);
 }