After node-pluginhost mixes PluginHost into Node, update all Node._instances

Consider:
```js
if (Y.one('#some')) {
  Y.use('tabview', function (Y) {
    new Y.TabView({srcNode: '#some', render: true});
  });
}
```
This breaks when `tabview` calls `this.get('contentBox').plug`.

The reason is that the contentBox `Node` is cached before `node-pluginhost` extends `Node`'s constructor (by mixing in `PluginHost`).

This doesn't happen if you use `YUI().use('tabview', function (Y) { ... });` (like you're supposed to), because the new sandbox has its own `Node._instances` cache.