Allow pattern case conditions in while statements.

[lrhn]

Dart allows if (e case p when c) ....
It should also allow while (e case p when c) ....

The true branch is the loop body, which is where the bound values are available.

An example use could be finding the last element of a linked list:

  while (cursor.next case var next?) cursor = next;

Generally if and while are mostly symmetric, and the behavior here can be emulated by:

while (true) {
  if (cursor.next case var next?) cursor = next;
  else break;
}

(Even if Dart doesn't make (e case p when c) a general boolean expression, it can still safely and meaningfully be allowed in a few more cases, mainly [conditional expression][#2664] and while conditions. I doesn't make sense for do/while since the true branch isn't dominated by the condition.)

[rrousselGit]

What about for?

for (...; something case ... ;  ...) {

}

If we can do it for while, I think it'd make sense here too.

[lrhn]

The for should work too. It's a little crowded syntactically, and it's not completely clear whether any bound variables are in scope for the increment-expressions or not. Still, the check being true dominates the body and increments, so it should probably just work for those.