Field populator needs access to object for ExtraField

[carterpage]

@ExtraField will typically need to access data from the object being populated.

For example

@ExtraField("recommendedAlbums")
public List<Album> getRecommendedAlbums()
{
    return someService.getRecommendedAbumsFor(USER_ID);
}

or

@ExtraField("total")
public int getTotal()
{
    return ONE_PART + ANOTHER_PART;
}

I don't see how to get these fields from within the populator.