code: @SuppressWarnings("unchecked")
    @VisibleForTesting
    JSType eval(Node ttlAst, ImmutableMap<String, JSType> typeVars,
        ImmutableMap<String, String> nameVars) {
      JSType result = evalInternal(ttlAst, new NameResolver(typeVars, nameVars));
      return result.isEmptyType() ? getUnknownType() : result;
    }
docs: Evaluates the type transformation expression and returns the resulting type.
  
  @param ttlAst The node representing the type transformation expression
  @param typeVars The environment containing the information about the type variables
  @param nameVars The environment containing the information about the name variables
  @return JSType The resulting type after the transformation