Triggered when the load begins.
First available in version 3.5 build 314.
Available only in ActionScript 3.
Event Parameter Properties:
bubbles - Boolean - Specifies if an event is a bubbling event.
cancelable - Boolean - Specifies if the behaviour can be canceled.
currentTarget - Object - The object that is currently handling the event.
target - Object - The event target.
Event; For details see Adobe documentation for URLLoader open event.