Add dummy dart:html library for documentation.

lib/html/doc/interface/ProgressElement.dartdoc

Index: lib/html/doc/interface/ProgressElement.dartdoc
diff --git a/lib/html/doc/interface/ProgressElement.dartdoc b/lib/html/doc/interface/ProgressElement.dartdoc
new file mode 100644
index 0000000000000000000000000000000000000000..a8a9f19958a91065e60a425489f745db70cd6459
--- /dev/null
+++ b/lib/html/doc/interface/ProgressElement.dartdoc
@@ -0,0 +1,25 @@
+// 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 HTMLProgressElement
+interface ProgressElement extends Element default _Elements {
+
+  ProgressElement();
+
+  /** @domName HTMLProgressElement.labels */
+  final NodeList labels;
+
+  /** @domName HTMLProgressElement.max */
+  num max;
+
+  /** @domName HTMLProgressElement.position */
+  final num position;
+
+  /** @domName HTMLProgressElement.value */
+  num value;
+}