Classes: Using class members

var p = Point(2, 2); 

// Set the value of the instance variable y.
p.y = 3; 

// Get the value of y. 
assert(p.y == 3); 

// Invoke distanceTo() on p. 
num distance = p.distanceTo(Point(4, 4));

// If p is non-null, set its y value to 4. 
p?.y = 4;