I have a plugin that adds some property to Vue instance.  

Then I can access this property inside components using `this.$plugin.prop`. How can I watch for its changes? I need to do something inside component based on `this.$plugin.prop` value but neither `watch` or `this.$watch` worked for me. I assume its because `watch` works in component context so I can't watch for variable outside component, for example 

```
    mounted() {
        this.$watch('$plugin.prop', val => console.log(val));
    }

```

doesn't work.  

What is the right way to accomplish this?