Add dummy dart:html library for documentation.

lib/html/doc/interface/HeadElement.dart

Index: lib/html/doc/interface/HeadElement.dart
diff --git a/lib/html/doc/interface/HeadElement.dart b/lib/html/doc/interface/HeadElement.dart
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index 0000000000000000000000000000000000000000..aea58049cf34aaee5ef618380612c2fe98a229dc
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+++ b/lib/html/doc/interface/HeadElement.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.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName HTMLHeadElement
+interface HeadElement extends Element default _Elements {
+
+  HeadElement();
+
+  /** @domName HTMLHeadElement.profile */
+  String profile;
+}