RFR(s): 8203670: unmodifiable List iterator() implementations should not be ListIterators

[Stuart Marks]

Hi all,

Please review this small changeset that ensures that calling iterator() on an 
unmodifiable List (from List.of, et. al.) returns an instance that cannot be 
downcast to ListIterator and used as such.

Bug:

    https://bugs.openjdk.java.net/browse/JDK-8203670

Webrev:

    http://cr.openjdk.java.net/~smarks/reviews/8203670/webrev.0/

Thanks,

s'marks