[Symbol.iterator] function appears to call #next one-too-many times without returning

[T99]

For some reason it appears that in the StringLineIterator class the #next function is called one-too-many times which results in the required type-check found here.

When used with a for ... of ([Symbol.iterator]) construct, there is always one final call to #next that does not return content to the iterator but that results in undefined being returned which was causing the call to String#trim() to throw a runtime error.