function modifierStateGetter(keyArg) {
	  var syntheticEvent = this;
	  var nativeEvent = syntheticEvent.nativeEvent;
	  if (nativeEvent.getModifierState) {
	    return nativeEvent.getModifierState(keyArg);
	  }
	  var keyProp = modifierKeyToProp[keyArg];
	  return keyProp ? !!nativeEvent[keyProp] : false;
	}
Translation from modifier key to the associated property in the event.
@see http://www.w3.org/TR/DOM-Level-3-Events/#keys-Modifiers