length property
The number of elements in this Iterable.
Counting all elements may involve iterating through all elements and can
therefore be slow.
Some iterables have a more efficient way to find the number of elements.
These must override the default implementation of length.
Implementation
int get length {
  assert(this is! EfficientLengthIterable);
  int count = 0;
  Iterator<Object?> it = iterator;
  while (it.moveNext()) {
    count++;
  }
  return count;
}