Recompute `entries_.length` on every iteration.
Most recent ES6 spec says that on every `next` call of MapIterator the number of elements in entries should be recomputed. See 23.1.5.2.1 10
I'm less sure about `Map.prototype.forEach` but unlike Array.prototype.forEach the spec doesn't specify that `len` should be cached. See 23.1.3.5