Add some sweet XML classes.

client/html/src/XMLDocument.dart

Index: client/html/src/XMLDocument.dart
diff --git a/client/html/src/XMLDocument.dart b/client/html/src/XMLDocument.dart
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+// 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 XMLDocument extends Document, XMLElement
+    default XMLDocumentWrappingImplementation {
+
+  /** WARNING: Currently this doesn't work on Dartium (issue 649). */
+  XMLDocument.xml(String xml);
+
+  XMLDocument clone(bool deep);
+}