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

tools/dom/src/dart2js_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!
  */
+@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 14 matching lines @@
 
   /** Accessor to the underlying keyCode value is the parent event. */
   int get _realKeyCode => JS('int', '#.keyCode', _parent);
 
   /** Accessor to the underlying charCode value is the parent event. */
   int get _realCharCode => JS('int', '#.charCode', _parent);
 
   /** Accessor to the underlying altKey value is the parent event. */
   bool get _realAltKey => JS('bool', '#.altKey', _parent);
 
+  /** Shadows on top of the parent's currentTarget. */
+  EventTarget _currentTarget;
+
   /** 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;
+    _currentTarget = _parent.currentTarget;
+  }
+
+  /** Programmatically create a new KeyEvent (and KeyboardEvent). */
+  factory 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, EventTarget currentTarget}) {
+    if (view == null) {
+      view = window;
+    }
+
+    var eventObj;
+    // In these two branches we create an underlying native KeyboardEvent, but
+    // we set it with our specified values. Because we are doing custom setting
+    // of certain values (charCode/keyCode, etc) only in this class (as opposed
+    // to KeyboardEvent) and the way we set these custom values depends on the
+    // type of underlying JS object, we do all the contruction for the
+    // underlying KeyboardEvent here.
+    if (canUseDispatchEvent) {
+      // Currently works in everything but Internet Explorer.
+      eventObj = new Event.eventType('Event', type,
+          canBubble: canBubble, cancelable: cancelable);
+
+      JS('void', '#.keyCode = #', eventObj, keyCode);
+      JS('void', '#.which = #', eventObj, keyCode);
+      JS('void', '#.charCode = #', eventObj, charCode);
+
+      JS('void', '#.keyLocation = #', eventObj, keyLocation);
+      JS('void', '#.ctrlKey = #', eventObj, ctrlKey);
+      JS('void', '#.altKey = #', eventObj, altKey);
+      JS('void', '#.shiftKey = #', eventObj, shiftKey);
+      JS('void', '#.metaKey = #', eventObj, metaKey);
+      JS('void', '#.altGraphKey = #', eventObj, altGraphKey);
+    } else {
+      // Currently this works on everything but Safari. Safari throws an
+      // "Attempting to change access mechanism for an unconfigurable property"
+      // TypeError when trying to do the Object.defineProperty hack, so we avoid
+      // this branch if possible.
+      // Also, if we want this branch to work in FF, we also need to modify
+      // _initKeyboardEvent to also take charCode and keyCode values to
+      // initialize initKeyEvent.
+
+      eventObj = new Event.eventType('KeyboardEvent', type,
+          canBubble: canBubble, cancelable: cancelable);
+
+      // Chromium Hack
+      JS('void', "Object.defineProperty(#, 'keyCode', {"
+          "  get : function() { return this.keyCodeVal; } })",  eventObj);
+      JS('void', "Object.defineProperty(#, 'which', {"
+          "  get : function() { return this.keyCodeVal; } })",  eventObj);
+      JS('void', "Object.defineProperty(#, 'charCode', {"
+          "  get : function() { return this.charCodeVal; } })",  eventObj);
+
+      var keyIdentifier = _convertToHexString(charCode, keyCode);
+      eventObj._initKeyboardEvent(type, canBubble, cancelable, view,
+          keyIdentifier, keyLocation, ctrlKey, altKey, shiftKey, metaKey,
+          altGraphKey);
+      JS('void', '#.keyCodeVal = #', eventObj, keyCode);
+      JS('void', '#.charCodeVal = #', eventObj, charCode);
+    }
+    // Tell dart2js that it smells like a KeyboardEvent!
+    var interceptor = new KeyboardEvent._private();

[sra1, 2013/10/07 17:47:11]

Add: TODO(13873): Use better way to get interceptor and remove _private()
constructors.

[Emily Fortuna, 2013/10/07 18:43:04]

Done.

+    var record = makeLeafDispatchRecord(interceptor);

[sra1, 2013/10/07 17:47:11]

We could make 'record' a static final so we create the dispatch record only
once.

static final _KeyboardEventDispatchRecord = _makeRecord();
_makeRecord() {
  var interceptor = new KeyboardEvent._private();
  return makeLeafDispatchRecord(interceptor);
}

setDispatchProperty(eventObj, _KeyboardEventDispatchRecord);

[Emily Fortuna, 2013/10/07 18:43:04]

Done.

+    setDispatchProperty(eventObj, record);
+
+    var keyEvent = new KeyEvent.wrap(eventObj);
+    if (keyEvent._currentTarget == null) {
+      keyEvent._currentTarget = currentTarget == null ? window : currentTarget;
+    }
+    return keyEvent;
+  }
+
+    // Currently known to work on all browsers but IE.
+  static bool get canUseDispatchEvent =>
+      JS('bool', '(typeof document.body.dispatchEvent == "function")'
+      '&& document.body.dispatchEvent.length > 0');

[sra1, 2013/10/07 17:47:11]

Better to have an argument that is multiline not share a line with another
argument:

    JS('bool',
        '(typeof document.body.dispatchEvent == "function")'
        ' && document.body.dispatchEvent.length > 0');

[Emily Fortuna, 2013/10/07 18:43:04]

Done.

+
+  /** The currently registered target for this event. */
+  EventTarget get currentTarget => _currentTarget;
+
+  // This is an experimental method to be sure.
+  static String _convertToHexString(int charCode, int keyCode) {
+    if (charCode != -1) {
+      var hex = charCode.toRadixString(16); // Convert to hexadecimal.
+      StringBuffer sb = new StringBuffer('U+');
+      for (int i = 0; i < 4 - hex.length; i++) sb.write('0');
+      sb.write(hex);
+      return sb.toString();
+    } else {
+      return KeyCode._convertKeyCodeToKeyName(keyCode);
+    }
   }
 
   // TODO(efortuna): If KeyEvent is sufficiently successful that we want to make
   // it the default keyboard event handling, move these methods over to Element.
   /** Accessor to provide a stream of KeyEvents on the desired target. */
   static EventStreamProvider<KeyEvent> keyDownEvent =
-    new _KeyboardEventHandler('keydown');
+      new _KeyboardEventHandler('keydown');
   /** Accessor to provide a stream of KeyEvents on the desired target. */
   static EventStreamProvider<KeyEvent> keyUpEvent =
-    new _KeyboardEventHandler('keyup');
+      new _KeyboardEventHandler('keyup');
   /** Accessor to provide a stream of KeyEvents on the desired target. */
   static EventStreamProvider<KeyEvent> keyPressEvent =
-    new _KeyboardEventHandler('keypress');
+      new _KeyboardEventHandler('keypress');
 
   /** True if the altGraphKey is pressed during this event. */
   bool get altGraphKey => _parent.altGraphKey;
   /** Accessor to the clipboardData available for this event. */
   DataTransfer get clipboardData => _parent.clipboardData;
   /** True if the ctrl key is pressed during this event. */
   bool get ctrlKey => _parent.ctrlKey;
   int get detail => _parent.detail;
   /**
    * Accessor to the part of the keyboard that the key was pressed from (one of
@@ skipping 20 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.");
   }
 }