Add dummy dart:html library for documentation.

lib/html/doc/interface/File.dartdoc

Index: lib/html/doc/interface/File.dartdoc
diff --git a/lib/html/doc/interface/File.dartdoc b/lib/html/doc/interface/File.dartdoc
new file mode 100644
index 0000000000000000000000000000000000000000..e11e9460fd2aba00369f1cfc13ed96c4a1cb5128
--- /dev/null
+++ b/lib/html/doc/interface/File.dartdoc
@@ -0,0 +1,20 @@
+// 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 File
+interface File extends Blob {
+
+  /** @domName File.lastModifiedDate */
+  final Date lastModifiedDate;
+
+  /** @domName File.name */
+  final String name;
+
+  /** @domName File.webkitRelativePath */
+  final String webkitRelativePath;
+}