Add some sweet XML classes.

client/html/src/XMLElement.dart

+// Copyright (c) 2012, 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 XMLElement extends Element default XMLElementWrappingImplementation {
+
+  XMLElement.tag(String tag);
+  XMLElement.xml(String tag);
+
+  XMLElement get firstElementChild();
+
+  XMLElement get lastElementChild();
+
+  XMLElement get nextElementSibling();
+
+  XMLElement get previousElementSibling();
+
+  XMLElement get offsetParent();

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

Why is there a user visible XMLElement interface? This seems confusing as its
inverted the normal element interface hierarchy.

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

See my reply to the overall CL comment.

+
+  XMLElement get parent();
+
+  XMLElement insertAdjacentElement([String where, XMLElement element]);
+
+  XMLElement query(String selectors);
+
+  XMLElement clone(bool deep);
+}