code: JSType eval(Node ttlAst, ImmutableMap<String, JSType> typeVars) {
      return eval(ttlAst, typeVars, ImmutableMap.of());
    }
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
  @return JSType The resulting type after the transformation