Add dummy dart:html library for documentation.

lib/html/doc/interface/Entity.dart

Index: lib/html/doc/interface/Entity.dart
diff --git a/lib/html/doc/interface/Entity.dart b/lib/html/doc/interface/Entity.dart
new file mode 100644
index 0000000000000000000000000000000000000000..8061e6fea3f20e9fdf383d9a1afe289a9102bfa4
--- /dev/null
+++ b/lib/html/doc/interface/Entity.dart
@@ -0,0 +1,18 @@
+// 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 Entity
+interface Entity extends Node {
+
+  /** @domName Entity.notationName */
+  final String notationName;
+
+  /** @domName Entity.publicId */
+  final String publicId;
+
+  /** @domName Entity.systemId */
+  final String systemId;
+}