Add dummy dart:html library for documentation.

lib/html/doc/interface/ContentElement.dart

Index: lib/html/doc/interface/ContentElement.dart
diff --git a/lib/html/doc/interface/ContentElement.dart b/lib/html/doc/interface/ContentElement.dart
new file mode 100644
index 0000000000000000000000000000000000000000..447c37cdcec8299ee0cc82e98285e24a67c4eec2
--- /dev/null
+++ b/lib/html/doc/interface/ContentElement.dart
@@ -0,0 +1,12 @@
+// 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 HTMLContentElement
+interface ContentElement extends Element {
+
+  /** @domName HTMLContentElement.select */
+  String select;
+}