evaluate method
Evaluates the expression for a given value of x.
This method returns the value of the expression when evaluated at x.
If x is not provided, the method should return the general form of the
expression or a representative value.
Returns:
- A 
dynamicrepresenting the evaluated value of the expression. 
Implementation
@override
dynamic evaluate([dynamic arg]) {
  var obj = object.evaluate(arg);
  return _getMember(obj, property.name);
}