`this` no longer references JavaScript object when implementing Java interface

When implementing a Java interface in JavaScript, `this` can no longer be used to reference the associated JavaScript object.

Example:
```js
var supplier = new java.util.function.Supplier({
    value: "Test",
    supply: function() {
        return this.value;
    }
});

supplier.supply();
```

Rhino versions before 1.9.0 would return the string `Test`, but since 1.9.0, `undefined` is returned.