bool operator ==(other)

The equality operator.

The default behavior for all Objects is to return true if and only if this and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

  • Total: It must return a boolean for all arguments. It should never throw or return null.

  • Reflexive: For all objects o, o == o must be true.

  • Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

  • Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so equality of two objects should not change over time, or at least only change if one of the objects was modified.

If a subclass overrides the equality operator it should override the hashCode method as well to maintain consistency.

Source 

bool operator ==(other) {
  if (other is !Rectangle) return false;
  return left == other.left && top == other.top && right == other.right &&
      bottom == other.bottom;
}