static findMissingKeys_(map, keys, missing) {
    for (const key of keys) {
      // Check if the value is missing so that we are sure that it does not
      // have a value. We get the value and not just |has| so that a null or
      // empty string will fail this check.
      if (!map.get(key)) {
        missing.add(key);
      }
    }
  }
Go through a map and add all the keys that are in |keys| but not in
|map| to |missing|.

@param {!Map<string, string>} map
@param {!Iterable<string>} keys
@param {!Set<string>} missing
@private