anchor.diff:  /// Throws an [Exception] if an invalid instruction is found.
   int partTwo(String instructions) {
     var floor = 0;
  -  for (var i = 0; i < instructions.length - 1; i++) {
  +
  +  var i = 0;
  +  while (i < instructions.length && floor != -1) {
       switch (instructions[i]) {
         case '(':
           floor++;
  
positive.code: /// Throws an [Exception] if an invalid instruction is found.
  int partOne(String instructions) {
    var floor = 0;
    for (var i = 0; i < instructions.length - 1; i++) {
      switch (instructions[i]) {
        case '(':
          floor++;
  
negative.code:       case ')':
          floor--;
          break;
        case '\n':
          break;
        default:
          throw 'Invalid instruction at index $i: \'${instructions[i]}\'.';
      }
    }
  
    return floor;
  }
  
  