[cfe] Surprising error message for expression in guard

[eernstg]

Consider the following program:

void main() {
  int d = 1;
  var x = false;
  switch (d) {
    case 1 when x = true:
      print('OK!');
  }
}

This program is rejected by the CFE (3.0.0-366.0.dev) with a surprising error message:

lib/main.dart:6:19:
Error: Not a constant expression.
    case 1 when x = true:
                  ^

This error message is surprising because there is no requirement that the expression in a guard must be constant (indeed, that wouldn't make sense).