Invalid type error for map for-of loop

[arv]

I'm seeing cases where the type of the key of a Map is flagged as the type of the key|value

/**
 * @param {!Map<string, number>} m
 */
function hello(m) {
  for (const [k, v] of m) {
    g(k) // k is incorrectly flagged as (string|number)
  }
}

For repro see.

https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540formatting%2520pretty_print%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250A%252F%252F%2520ADD%2520YOUR%2520CODE%2520HERE%250A%252F**%250A%2520*%2520%2540param%2520%257B!Map%253Cstring%252C%2520number%253E%257D%2520m%250A%2520*%252F%250Afunction%2520hello(m)%2520%257B%250A%2520%2520for%2520(const%2520%255Bk%252C%2520v%255D%2520of%2520m)%2520%257B%250A%2520%2520%2520%2520g(k)%250A%2520%2520%257D%250A%257D%250A%250A%252F**%250A%2520*%2520%2540param%2520%257Bstring%257D%2520s%250A%2520*%252F%250Afunction%2520g(s)%2520%257B%257D%250Ahello(new%2520Map())%253B%250A%250A

[arv]

I guess this comes down to the Map iterator returning ([K, V], [K, V] ...) and JSCompiler does not know better than to treat these as Array<K|V>. Maybe there needs to be some special case for this since this pattern is super useful.

[arv]

Related to #379

@concavelenz 👋 I saw your todo in the externs file...