This merge might not be enough. For example, if the custom meta added before the loader gets injected has a custom group, e.g:

```
YUI.Env[Y.version].groups = {
    foo: {
        modules = {
            mod1: {
                fullpath: '../assets/mod1.js',
                attributes: {
                    id: 'early-module-meta'
                }
            }
        }
    }
};
```

it will be removed by the merge operation because `meta.js` comes with its own `groups` definition.

This is probably an issue for `modules` and `groups` only AFAIK, but `skin` and `patterns` are suspicious as well. We could try to mix them if they exist to avoid a perf regression compared to the current implementation.