Add dummy dart:html library for documentation.

lib/html/doc/interface/CSSValueList.dartdoc

Index: lib/html/doc/interface/CSSValueList.dartdoc
diff --git a/lib/html/doc/interface/CSSValueList.dartdoc b/lib/html/doc/interface/CSSValueList.dartdoc
new file mode 100644
index 0000000000000000000000000000000000000000..38234a3975b45576b16c1ca8c20bf6b0eb640417
--- /dev/null
+++ b/lib/html/doc/interface/CSSValueList.dartdoc
@@ -0,0 +1,17 @@
+// 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 CSSValueList
+interface CSSValueList extends CSSValue {
+
+  /** @domName CSSValueList.length */
+  final int length;
+
+  /** @domName CSSValueList.item */
+  CSSValue item(int index);
+}