code: public JSType getInstantiatedTypeArgument(JSType supertype) {
      TemplateType templateType =
          Iterables.getOnlyElement(supertype.getTemplateTypeMap().getTemplateKeys());
      return getTemplateTypeMap().getResolvedTemplateType(templateType);
    }
docs: Returns the template type argument in this type's map corresponding to the supertype's template
  parameter, or the UNKNOWN_TYPE if the supertype template key is not present.
  
  <p>Note: this only supports arguments that have a singleton list of template keys, and will
  throw an exception for arguments with zero or multiple or template keys.