Add dummy dart:html library for documentation.

lib/html/doc/interface/TableSectionElement.dartdoc

+// 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.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName HTMLTableSectionElement
+interface TableSectionElement extends Element {
+
+  /** @domName HTMLTableSectionElement.align */
+  String align;
+
+  /** @domName HTMLTableSectionElement.ch */
+  String ch;
+
+  /** @domName HTMLTableSectionElement.chOff */
+  String chOff;
+
+  /** @domName HTMLTableSectionElement.rows */
+  final HTMLCollection rows;
+
+  /** @domName HTMLTableSectionElement.vAlign */
+  String vAlign;
+
+  /** @domName HTMLTableSectionElement.deleteRow */
+  void deleteRow(int index);
+
+  /** @domName HTMLTableSectionElement.insertRow */
+  Element insertRow(int index);
+}