Clarify list behavior in concurrent modification cases

[ghost]

Should we throw a ConcurrentModificationException if a list is modified in such a way that an active iteration becomes corrupted?

See implementation of DoubleLinkedQueue.

[DartBot]

This comment was originally written by ngeoffray@google.com

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When designing the libraries, we decided not to do defensive checks in the iterators and iteration methods in case the collection is changed while iterating. That may be poorly documented though.

The rationale behind that decision is that we expect most uses of iteration methods to not require these checks. And we don't want to pay the price of runtime checks due to a minority of uses.

So when a concurrent modification happens, the behavior is unspecified.

[madsager]

Comment updated to state that the behavior is unspecified if the object iterated over is changed during iteration.

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Added Fixed label.