Unmerging HTMLDocument from Document.

tests/html/xmldocument_2_test.dart

+library XmlDocument2Test;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
+
+main() {
+  useHtmlConfiguration();
+  test('Document.query', () {
+    Document doc = new DOMParser().parseFromString(
+    '''<ResultSet>
+         <Row>A</Row>
+         <Row>B</Row>
+         <Row>C</Row>
+       </ResultSet>''','text/xml');
+
+    var rs = doc.query('ResultSet');
+    expect(rs, isNotNull);
+  });
+}