Add dummy dart:html library for documentation.

lib/html/doc/interface/LinkElement.dartdoc

Index: lib/html/doc/interface/LinkElement.dartdoc
diff --git a/lib/html/doc/interface/LinkElement.dartdoc b/lib/html/doc/interface/LinkElement.dartdoc
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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.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName HTMLLinkElement
+interface LinkElement extends Element default _Elements {
+
+  LinkElement();
+
+  /** @domName HTMLLinkElement.charset */
+  String charset;
+
+  /** @domName HTMLLinkElement.disabled */
+  bool disabled;
+
+  /** @domName HTMLLinkElement.href */
+  String href;
+
+  /** @domName HTMLLinkElement.hreflang */
+  String hreflang;
+
+  /** @domName HTMLLinkElement.media */
+  String media;
+
+  /** @domName HTMLLinkElement.rel */
+  String rel;
+
+  /** @domName HTMLLinkElement.rev */
+  String rev;
+
+  /** @domName HTMLLinkElement.sheet */
+  final StyleSheet sheet;
+
+  /** @domName HTMLLinkElement.sizes */
+  DOMSettableTokenList sizes;
+
+  /** @domName HTMLLinkElement.target */
+  String target;
+
+  /** @domName HTMLLinkElement.type */
+  String type;
+}