Add dummy dart:html library for documentation.

lib/html/doc/interface/RGBColor.dart

Index: lib/html/doc/interface/RGBColor.dart
diff --git a/lib/html/doc/interface/RGBColor.dart b/lib/html/doc/interface/RGBColor.dart
new file mode 100644
index 0000000000000000000000000000000000000000..561e41fdb8336b9c9ab2ec40dc52fd4d81943845
--- /dev/null
+++ b/lib/html/doc/interface/RGBColor.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 RGBColor
+interface RGBColor {
+
+  /** @domName RGBColor.blue */
+  final CSSPrimitiveValue blue;
+
+  /** @domName RGBColor.green */
+  final CSSPrimitiveValue green;
+
+  /** @domName RGBColor.red */
+  final CSSPrimitiveValue red;
+}