when promoting a type parameter, promote the upper bound

[jmesserly]

given:

void fn/*<T>*/(/*=T*/ object) {
  if (object is String) print(object.substring(1));
}

we will now promote to String (fix for #26965) but even better would be to promote to T extends String

original context here: #26965 (comment)

[jmesserly]

This could probably be accomplished by moving the bound to the TypeParameterType. That way you could have multiple TypeParameterTypes that point at the same TypeParameterElement.

[jmesserly]

Oh for what it's worth ... having type parameters that can track bounds (lower and upper) is essentially what we need for generic method inference. So this might become a lot more tractable when we address https://github.com/dart-lang/sdk/issues/25490 (unified upwards & downwards inference)

[jmesserly]

Oh wow ... I just stumbled across 66e1a1d ... apparently we have a thing called TypeParameterMember. I wonder if it could be adapted to this use case here.

[jmesserly]

This shouldn't actually be hard to fix now. Taking a look.

[jmesserly]

CL has been out for a while now: https://codereview.chromium.org/2375373002, I'm adding some tests based on the feedback. There's still some problematic behavior due to how operator== works for TypeParameterTypeImpl. So it'll be worth thinking over if we want this fix.