Add dummy dart:html library for documentation.

lib/html/doc/interface/ProgressEvent.dart

Index: lib/html/doc/interface/ProgressEvent.dart
diff --git a/lib/html/doc/interface/ProgressEvent.dart b/lib/html/doc/interface/ProgressEvent.dart
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+++ b/lib/html/doc/interface/ProgressEvent.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 ProgressEvent
+interface ProgressEvent extends Event {
+
+  /** @domName ProgressEvent.lengthComputable */
+  final bool lengthComputable;
+
+  /** @domName ProgressEvent.loaded */
+  final int loaded;
+
+  /** @domName ProgressEvent.total */
+  final int total;
+}