Add dummy dart:html library for documentation.

lib/html/doc/interface/ProgressElement.dart

Index: lib/html/doc/interface/ProgressElement.dart
diff --git a/lib/html/doc/interface/ProgressElement.dart b/lib/html/doc/interface/ProgressElement.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 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;
+}