trimStart method
Implementation
void trimStart(int count) {
  if (count > _length) count = _length;
  _startIndex += count;
  _startIndex %= _array.length;
  _length -= count;
}
void trimStart(int count) {
  if (count > _length) count = _length;
  _startIndex += count;
  _startIndex %= _array.length;
  _length -= count;
}