Extend type test expression to accept instance of Type in addition to type name

[scheglov]

I.e. now this works:
print(v is MyObject);

But this - not.
Type t = MyObject;
print(v is t);

v.runtimeType == MyObject is not enough, I'd like to check also subclasses of MyObject.

[gbracha]

We are considering this for constant variables.

---

Added Accepted label.

[scheglov]

Hm... Constant variables is too restrictive for my needs.
I need it to make work code like this:

  Element getAncestor(Type elementType) {
    Element ancestor = enclosingElement;
    while (ancestor != null && ancestor is! elementType) {
      ancestor = ancestor.enclosingElement;
    }
    return ancestor;
  }

Where "elementType" is something like "MethodElement" and actual types of "ancestor" and "enclosingElement" are "MethodElementImpl", "CompilationUnitImpl", etc.

[lrhn]

Removed Type-Defect label.
Added Type-Enhancement label.

[munificent]

Marking this as a dupe of the newer bug (#27680) since that one has a little more discussion.