Investigate terminate state after infinite loop with return

[dcodeIO]

While working on #1129 I noticed that code like the following still wants a return statement

  get file(): File {
    var current: Element = this;
    do {
      current = current.parent;
      if (current.kind == ElementKind.FILE) return <File>current;
    } while (true);
    return <File>unreachable(); // here
  }

not noticing that control flow cannot actually fall through. Pinning here so we don't forget.