Otherwise we crash the interpreter when an exception is thrown during
evaluation of the while or do/while test expression - which is easily
caused by a ReferenceError - e.g.:
while (someUndefinedVariable) {
// ...
}
Finally use Symbol.iterator protocol in language features :) currently
only used in for-of loops and spread expressions, but will have more
uses later (Maps, Sets, Array.from, etc).
