function Foo(x)
{
this.f = x + 10;
}
var x = new Foo(0);
assertEq(10, eval("x.f"));
called = false;
Object.defineProperty(Foo.prototype, 'f', {set: function() { called = true; }});
var y = new Foo(0);
assertEq(10, eval("x.f"));
assertEq(undefined, eval("y.f"));
assertEq(called, true);