Add dummy dart:html library for documentation.

lib/html/doc/interface/WebGLCompressedTextureS3TC.dartdoc

+// 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 WebGLCompressedTextureS3TC
+interface WebGLCompressedTextureS3TC {
+
+  static final int COMPRESSED_RGBA_S3TC_DXT1_EXT = 0x83F1;
+
+  static final int COMPRESSED_RGBA_S3TC_DXT3_EXT = 0x83F2;
+
+  static final int COMPRESSED_RGBA_S3TC_DXT5_EXT = 0x83F3;
+
+  static final int COMPRESSED_RGB_S3TC_DXT1_EXT = 0x83F0;
+}