`shadowObservable` doesn't work on property of `ViewActivity`

[jcormont]

In the example below, the binding to foo doesn't seem to work:

class MyActivity extends PageViewActivity.with(
  UICell.with(
    UILabel.withText(bind("foo"))
  )
) {
  @shadowObservable("bar")
  get foo() { return this.bar + 1 }
  
  bar = 1

  // ...
}

The reason here is that foo is already observed in the class that results from PageViewActivity.with(...); this class is then extended, and the shadow observable property is added. At that point it is too late to change the behavior of the binding.

Resolution: in shadowObservable() function, check for observed/managed properties in prototypes and fail if there are any.

If there really must be a shadow observable property, it must be added on a class first, and then the class can be extended using .with(/* view */).