Compile-time error message about a non-joined variable usage doesn't state the reason clearly

[chloestefantsova]

Consider the following program. Compiling it results in an error: the variable with name a isn't captured by every head of the switch case, so it can't be used in the case body. However, the error message produced by the CFE isn't very helpful in this case, simply stating that a is undefined in the body.

test(dynamic x) {
  switch (x) {
    case final a when a > 0:
    case 0:
    case final a:
      return a; // Error.
    default:
      return null;
  }
}

// /tmp/asdf2.dart:6:14: Error: Undefined name 'a'.
//       return a;
//              ^