current property Null safety
The current element.
If the iterator has not yet been moved to the first element (moveNext has not been called yet), or if the iterator has been moved past the last element of the Iterable (moveNext has returned false), then current is unspecified. An Iterator may either throw or return an iterator specific default value in that case.
current getter should keep its value until the next call to
moveNext, even if an underlying collection changes.
After a successful call to
moveNext, the user doesn't need to cache
the current value, but can keep reading it from the iterator.
E get current;