Currently if I want to subclass the System loader I need to do something
like -

var newLoader = new Loader(System);
newLoader.fetch = function() {
  // ...
}

Effectively we're monkey-patching, which isn't pretty.

It would be nice to be able to do:

class newLoader extends System.constructor {
  constructor(loaderConfig) {
    this.baseURL = loaderConfig.baseURL;
    // ...
  }
  fetch: function() {
    super.fetch
  }
}

In order to allow this, we would need to first define a SystemLoader class,
and make System an instance of it.
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Currently if I want to subclass the System loader I need to do something
like -

```js
var newLoader = new Loader(System);
newLoader.fetch = function() {
  // ...
}
```

Effectively we're monkey-patching, which isn't pretty.

It would be nice to be able to do:

```js
class newLoader extends System.constructor {
  constructor(loaderConfig) {
    this.baseURL = loaderConfig.baseURL;
    // ...
  }
  fetch: function() {
    super.fetch
  }
}
```

In order to allow this, we would need to first define a SystemLoader class,
and make System an instance of it.