XML - Implement xml["@id"] style attribute accessor

[sushihangover]

Currently these access methods are supported:

• xml.@id
• xml@["id"]
• xml.attribute("id")

This style is not currently supported, but is allowed in AS3:

• xml["@id"]

  New Test: as/test-as-XML-Attr-Access-3.as

// Compiler options: -psstrict-
package {
    public class Foo {
        public static function Main():int {
                var xml:XML=<example id='123' color='blue'/>;

                var idAttrArrayIndexer:String = xml["@id"];
                trace(idAttrArrayIndexer);
                if (idAttrArrayIndexer != "123") return 1

                return 0;
        }
    }
}