title: ngFor trackBy function with custom parameters
body: <p>The trackBy function (e.g. in an ngFor) provides two arguments: index and item (from the collection being iterated over).  Is there a way to pass additional information (as parameters?) to th trackBy function?</p>
  
  <p>My case is that I might be iterating over a variety of types for each instance of my component (which contains the ngFor), with different identifying field names.  Ideally, I'd like to be able to pass a third parameter indicating which field in my item should be read.</p>