code: TemplateTypeMap extend(TemplateTypeMap other) {
      ImmutableList<JSType> resizedOtherValues = other.resizedToMatchKeys(other.templateValues);
      return registry.createTemplateTypeMap(
          concatImmutableLists(other.templateKeys, templateKeys),
          concatImmutableLists(resizedOtherValues, templateValues));
    }
docs: Extends this TemplateTypeMap with the contents of the specified map. UNKNOWN_TYPE will be used
  as the value for any missing values in the specified map.