In Dart how can I define a Arithmetic Operator as a Variable?

In dart I want to set a plus or minus operator as a variable.

var plusOrMinus;
    
    if (x >= y) {
      plusOrMinus = -;
    } else {
    plusOrMinus = +;
    }

How do I define plusOrMinus so that it can be used like this:

double width = 10.0 plusOrMinus 5.0;

>Solution :

Capture the operation as a closure:

final operation = x >= y ?
  (a, b) => a - b :
  (a, b) => a + b;

final width = operation(10.0, 5.0);