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

tools/dom/src/dartium_KeyEvent.dart

 /**
  * A custom KeyboardEvent that attempts to eliminate cross-browser
  * inconsistencies, and also provide both keyCode and charCode information
  * for all key events (when such information can be determined).
  *
  * KeyEvent tries to provide a higher level, more polished keyboard event
  * information on top of the "raw" [KeyboardEvent].
  *
  * This class is very much a work in progress, and we'd love to get information
  * on how we can make this class work with as many international keyboards as
  * possible. Bugs welcome!
  */
 part of html;
 
+@Experimental()
 class KeyEvent extends _WrappedEvent implements KeyboardEvent {
   /** The parent KeyboardEvent that this KeyEvent is wrapping and "fixing". */
   KeyboardEvent _parent;
 
   /** The "fixed" value of whether the alt key is being pressed. */
   bool _shadowAltKey;
 
   /** Caculated value of what the estimated charCode is for this event. */
   int _shadowCharCode;
 
@@ skipping 15 matching lines @@
   /** Accessor to the underlying keyCode value is the parent event. */
   int get _realKeyCode => _parent.keyCode;
 
   /** Accessor to the underlying charCode value is the parent event. */
   int get _realCharCode => _parent.charCode;
 
   /** Accessor to the underlying altKey value is the parent event. */
   bool get _realAltKey => _parent.altKey;
 
   /** Construct a KeyEvent with [parent] as the event we're emulating. */
-  KeyEvent(KeyboardEvent parent): super(parent) {
+  KeyEvent.wrap(KeyboardEvent parent): super(parent) {
     _parent = parent;
     _shadowAltKey = _realAltKey;
     _shadowCharCode = _realCharCode;
     _shadowKeyCode = _realKeyCode;
   }
 
+  /** Programmatically create a new KeyEvent (and KeyboardEvent). */
+   KeyEvent(String type,
+      {Window view, bool canBubble: true, bool cancelable: true, int keyCode: 0,
+      int charCode: 0, int keyLocation: 1, bool ctrlKey: false,
+      bool altKey: false, bool shiftKey: false, bool metaKey: false,
+      bool altGraphKey: false}) {
+     _parent = new KeyboardEvent(type, view: view, canBubble: canBubble,
+        cancelable: cancelable, keyLocation: keyLocation, ctrlKey: ctrlKey,
+        altKey: altKey, shiftKey: shiftKey, metaKey: metaKey, altGraphKey:
+        altGraphKey);
+      _shadowAltKey = altKey;
+      _shadowCharCode = charCode;
+      _shadowKeyCode = keyCode;
+   }
+
   /** Accessor to provide a stream of KeyEvents on the desired target. */
   static EventStreamProvider<KeyEvent> keyDownEvent =
     new _KeyboardEventHandler('keydown');
   /** Accessor to provide a stream of KeyEvents on the desired target. */
   static EventStreamProvider<KeyEvent> keyUpEvent =
     new _KeyboardEventHandler('keyup');
   /** Accessor to provide a stream of KeyEvents on the desired target. */
   static EventStreamProvider<KeyEvent> keyPressEvent =
     new _KeyboardEventHandler('keypress');
 
@@ skipping 29 matching lines @@
     throw new UnsupportedError("keyIdentifier is unsupported.");
   }
   void _initKeyboardEvent(String type, bool canBubble, bool cancelable,
       Window view, String keyIdentifier, int keyLocation, bool ctrlKey,
       bool altKey, bool shiftKey, bool metaKey,
       bool altGraphKey) {
     throw new UnsupportedError(
         "Cannot initialize a KeyboardEvent from a KeyEvent.");
   }
 }