Add constructors to all the event classes.

client/html/src/DeviceMotionEvent.dart

Index: client/html/src/DeviceMotionEvent.dart
diff --git a/client/html/src/DeviceMotionEvent.dart b/client/html/src/DeviceMotionEvent.dart
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+++ b/client/html/src/DeviceMotionEvent.dart
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+// Copyright (c) 2011, 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.
+
+interface DeviceMotionEvent extends Event factory DeviceMotionEventWrappingImplementation {
+
+  // TODO(nweiz): Add more arguments to the constructor when we support
+  // DeviceMotionEvent more thoroughly.
+  DeviceMotionEvent(String type, [bool canBubble, bool cancelable]);
+
+  num get interval();
+}