Add dummy dart:html library for documentation.

lib/html/doc/interface/XMLHttpRequestProgressEvent.dart

Index: lib/html/doc/interface/XMLHttpRequestProgressEvent.dart
diff --git a/lib/html/doc/interface/XMLHttpRequestProgressEvent.dart b/lib/html/doc/interface/XMLHttpRequestProgressEvent.dart
new file mode 100644
index 0000000000000000000000000000000000000000..dd2ab0583546054f7e7dec2b678df95692b699d4
--- /dev/null
+++ b/lib/html/doc/interface/XMLHttpRequestProgressEvent.dart
@@ -0,0 +1,15 @@
+// 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 XMLHttpRequestProgressEvent
+interface XMLHttpRequestProgressEvent extends ProgressEvent {
+
+  /** @domName XMLHttpRequestProgressEvent.position */
+  final int position;
+
+  /** @domName XMLHttpRequestProgressEvent.totalSize */
+  final int totalSize;
+}