Calling through a subtype can bind differently from the supertype

[munificent]

The same function called through two different static types can bind different parameters:

foo([int x], int y, [int z]) => "$x$y$z";
Function([int x], int y, int z) bar = foo;
print(foo(1, 2));  // 12null
print(bar(1, 2));  // null12

It "works", but that's not what I intended and I don't think it's what we want.