Can't access map instance on validators when value is set

[m-mujica]

Let's say you want to conditionally validate the presence of a can/map property

I'd expect I could write something like

Map.extend({
  define: {
    foo: {
      validate: {
        presence(value, attributes, attributeName) {
          return this.attr('mySpecialProperty') === mySpecialValue;
      }
    }
  }
});

See ansman/validate.js#33

The problem with that is:

1. this is null inside the presence function
2. The attributes argument is an object with the map instance properties when mapInstance.validate() is called, but when mySpecialProperty is set, attributes no longer contains the map properties, so we can't rely on that

Is there a way to implement conditional validation like this currently? what am I missing?