Flash CS3 Professional.
document.as3Dialect
Property; a string that describes the ActionScript 3.0 "dialect" being used in the specified document. The default value is "AS3". If you wish to allow prototype classes, as permitted in earlier ECMAScript specifications, set this value to "ES".
The following example specifies that the dialect being used in the current document is ECMAScript:
fl.getDocumentDOM().as3Dialect="ES";