We try to avoid accessing the element and its attributes directly -- using the map in @NgDirective helps our tooling and sets up a attribute observer (more important when Web Components start taking off).  (see https://github.com/angular/angular.dart/blob/master/lib/core_dom/directive.dart#L21)

But, I don't know how to implement these when-xx attributes.  Two options:
-> keep this code as is and deal with the bugs later
-> remove the when-xx and when-minus-xx attributes.  They are just sugar on top of 'when'.

Thoughts?  I don't have a strong opinion either way.