ABSTRACTS: Fix @:commutative on ops without a body

`@:commutative` was not working for operators without a body, e.g.:

````
package;

abstract Foo(Int) from Int to Int {
	@:commutative @:op(A+B) public static function add(lhs:Foo, rhs:Int);
}

class Main {
	public static function main() {
		var a : Foo = 1;
		trace( a + 1 );
		trace( 1 + a ); // failed: Cannot add Int and Foo
	}
}
````

This patch should fix the issue.