Fully polyfill KeyEvent so that you can programmatically create your own "keyboard" events.

tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate

 // 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.
 
 
 /**
  * An event that describes user interaction with the keyboard.
  *
  * The [type] of the event identifies what kind of interaction occurred.
  *
@@ skipping 20 matching lines @@
       bool altGraphKey: false}) {
     if (view == null) {
       view = window;
     }
     final e = document._createEvent("KeyboardEvent");
     e._initKeyboardEvent(type, canBubble, cancelable, view, "",
         keyLocation, ctrlKey, altKey, shiftKey, metaKey, altGraphKey);
     return e;
   }
 
+  /** 
+   * Used for KeyboardEvent polyfilling when programmatically constructing
+   * KeyEvents.
+   */
+  $CLASSNAME._private() : super._private();
+
   @DomName('KeyboardEvent.initKeyboardEvent')
   void _initKeyboardEvent(String type, bool canBubble, bool cancelable,
       Window view, String keyIdentifier, int keyLocation, bool ctrlKey,
       bool altKey, bool shiftKey, bool metaKey, bool altGraphKey) {
     if (JS('bool', 'typeof(#.initKeyEvent) == "function"', this)) {
       // initKeyEvent is only in Firefox (instead of initKeyboardEvent). It has
       // a slightly different signature, and allows you to specify keyCode and
       // charCode as the last two arguments, but we just set them as the default
       // since they can't be specified in other browsers.
       JS('void', '#.initKeyEvent(#, #, #, #, #, #, #, #, 0, 0)', this,
           type, canBubble, cancelable, view,
           ctrlKey, altKey, shiftKey, metaKey);
     } else {
       // initKeyboardEvent is for all other browsers.
       JS('void', '#.initKeyboardEvent(#, #, #, #, #, #, #, #, #, #, #)', this,
           type, canBubble, cancelable, view, keyIdentifier, keyLocation,
           ctrlKey, altKey, shiftKey, metaKey, altGraphKey);
     }
   }
 
   @DomName('KeyboardEvent.keyCode')
   int get keyCode => _keyCode;
 
   @DomName('KeyboardEvent.charCode')
   int get charCode => _charCode;
 $!MEMBERS
 }