We now have a best-effort check+fail if a map is concurrently accessed by two goroutines, one of which is a write. (This is a bug in the user's code.) That check is incomplete in the the iterator code path.

Iterators should be considered a read in all circumstances. The check in mapaccessK only happens when iterating during a grow. That check should be moved to mapiternext so it executes for all iterators.

@rsc @josharian