Add dummy dart:html library for documentation.

lib/html/doc/interface/DetailsElement.dart

Index: lib/html/doc/interface/DetailsElement.dart
diff --git a/lib/html/doc/interface/DetailsElement.dart b/lib/html/doc/interface/DetailsElement.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9e76e792ce053f89170caf7c216437fe7a8a4a2b
--- /dev/null
+++ b/lib/html/doc/interface/DetailsElement.dart
@@ -0,0 +1,14 @@
+// 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 HTMLDetailsElement
+interface DetailsElement extends Element default _Elements {
+
+  DetailsElement();
+
+  /** @domName HTMLDetailsElement.open */
+  bool open;
+}