RuneIterator.at constructor
Create an iterator positioned before the index
th code unit of the string.
When created, there is no current value.
A moveNext will use the rune starting at index
the current value,
and a movePrevious will use the rune ending just before index
as
the current value.
The index
position must not be in the middle of a surrogate pair.
Implementation
RuneIterator.at(String string, int index)
: string = string,
_position = index,
_nextPosition = index {
RangeError.checkValueInInterval(index, 0, string.length);
_checkSplitSurrogate(index);
}