 produce
different elements, and may change the order of existing elements.
A <a href="dart-core/List-class.html">List</a> specifies its iteration order precisely,
so modifying the list changes the iteration order predictably.
A hash-based <a href="dart-core/Set-class.html">Set</a> may change its iteration order completely
when adding a new element to the set.</p>
<p>Modifying the underlying collection after creating the new iterator
may cause an error the next time <a href="dart-core/Iterator/moveNext.html