testClass.x could not be evaluated

package;
import flash.display.Sprite;
class Main extends Sprite
{
public function new()
{
super();
var testClass: TestClass = new TestClass();
testClass.x = 20;
trace(testClass.x);
}
}
package;
class TestClass
{
private var _x: Int = 0;
public var x(get, set):Int;
public function new()
{
}
private function get_x() {
return this._x;
}
private function set_x(value) {
return this._x = value;
}
}