code: boolean expectAutoboxesToIterable(Node n, JSType type, String msg) {
      // Note: we don't just use JSType.autobox() here because that removes null and undefined.
      // We want to keep null and undefined around.
      if (type.isUnionType()) {
        for (JSType alt : type.toMaybeUnionType().getAlternates()) {
          alt = alt.isBoxableScalar() ? alt.autoboxesTo() : alt;
          if (!alt.isSubtypeOf(getNativeType(ITERABLE_TYPE))) {
            mismatch(n, msg, type, ITERABLE_TYPE);
            return false;
          }
        }
  
      } else {
        JSType autoboxedType = type.isBoxableScalar() ? type.autoboxesTo() : type;
        if (!autoboxedType.isSubtypeOf(getNativeType(ITERABLE_TYPE))) {
          mismatch(n, msg, type, ITERABLE_TYPE);
          return false;
        }
      }
      return true;
    }
docs: Expect the type to autobox to be an Iterable.
  
  @return True if there was no warning, false if there was a mismatch.