on-watch rule triggers on `return`

`return` statement must be treated equivalent to an assignment for the purpose of the **on-watch** rule.

My use case involves returning a function that ends with `return $rootScope.$on(...)` from a service into a controller. I can use the return value with the `'$destroy'` event. I know the workaround is to place `$rootscope.$on()` in the controller but with my method I can avoid a `$rootScope` DI in the controller.