Add dummy dart:html library for documentation.

lib/html/doc/interface/DOMMimeType.dartdoc

Index: lib/html/doc/interface/DOMMimeType.dartdoc
diff --git a/lib/html/doc/interface/DOMMimeType.dartdoc b/lib/html/doc/interface/DOMMimeType.dartdoc
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+++ b/lib/html/doc/interface/DOMMimeType.dartdoc
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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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName DOMMimeType
+interface DOMMimeType {
+
+  /** @domName DOMMimeType.description */
+  final String description;
+
+  /** @domName DOMMimeType.enabledPlugin */
+  final DOMPlugin enabledPlugin;
+
+  /** @domName DOMMimeType.suffixes */
+  final String suffixes;
+
+  /** @domName DOMMimeType.type */
+  final String type;
+}