Add dummy dart:html library for documentation.

lib/html/doc/interface/DeviceOrientationEvent.dart

Index: lib/html/doc/interface/DeviceOrientationEvent.dart
diff --git a/lib/html/doc/interface/DeviceOrientationEvent.dart b/lib/html/doc/interface/DeviceOrientationEvent.dart
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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: Do not edit - generated code.
+
+/// @domName DeviceOrientationEvent
+interface DeviceOrientationEvent extends Event {
+
+  /** @domName DeviceOrientationEvent.absolute */
+  final bool absolute;
+
+  /** @domName DeviceOrientationEvent.alpha */
+  final num alpha;
+
+  /** @domName DeviceOrientationEvent.beta */
+  final num beta;
+
+  /** @domName DeviceOrientationEvent.gamma */
+  final num gamma;
+
+  /** @domName DeviceOrientationEvent.initDeviceOrientationEvent */
+  void initDeviceOrientationEvent(String type, bool bubbles, bool cancelable, num alpha, num beta, num gamma, bool absolute);
+}