ViewPorts should expose information about views scheduled to be created

[jbdeboer]

I have a decorator that modifies it state if it has a View in its ViewPort. e.g.

bool get hasViews => _viewPort.views.isNotEmpty;

Since Views are inserted during the domWrite phase, this getter will flip values during that phase causing a Caught Observer reaction functions should not change model. error.

ViewPort should expose the set of Views that are scheduled to be created, so I can watch that field instead and update my state at the correct time. (Now, I have a 0 duration timer hack).

[naomiblack]

We should revisit this in January.