code: @SuppressWarnings("unchecked") // safe because we only read, not write
    public IterableSubject valuesForKey(@NullableDecl Object key) {
      return check("valuesForKey(%s)", key).that(((Multimap<Object, Object>) actual()).get(key));
    }
docs: Returns a context-aware {@link Subject} for making assertions about the values for the given
  key within the {@link Multimap}.
  
  <p>This method performs no checks on its own and cannot cause test failures. Subsequent
  assertions must be chained onto this method call to test properties of the {@link Multimap}.