Add dummy dart:html library for documentation.

lib/html/doc/interface/TableColElement.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 HTMLTableColElement
+interface TableColElement extends Element default _Elements {
+
+  TableColElement();
+
+  /** @domName HTMLTableColElement.align */
+  String align;
+
+  /** @domName HTMLTableColElement.ch */
+  String ch;
+
+  /** @domName HTMLTableColElement.chOff */
+  String chOff;
+
+  /** @domName HTMLTableColElement.span */
+  int span;
+
+  /** @domName HTMLTableColElement.vAlign */
+  String vAlign;
+
+  /** @domName HTMLTableColElement.width */
+  String width;
+}