Add dummy dart:html library for documentation.

lib/html/doc/interface/Coordinates.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 Coordinates
+interface Coordinates {
+
+  /** @domName Coordinates.accuracy */
+  final num accuracy;
+
+  /** @domName Coordinates.altitude */
+  final num altitude;
+
+  /** @domName Coordinates.altitudeAccuracy */
+  final num altitudeAccuracy;
+
+  /** @domName Coordinates.heading */
+  final num heading;
+
+  /** @domName Coordinates.latitude */
+  final num latitude;
+
+  /** @domName Coordinates.longitude */
+  final num longitude;
+
+  /** @domName Coordinates.speed */
+  final num speed;
+}