code: public static DictImpl forProviderMap(
        Map<String, ? extends SoyValueProvider> providerMap, RuntimeMapTypeTracker.Type mapType) {
      return new DictImpl(providerMap, mapType);
    }
docs: Creates a SoyDict implementation for a particular underlying provider map.
  
  <p>The map may be mutable, but will not be mutated by the DictImpl.