InjectionIntoFrame.js

var myiframe = document.getElementById('#frame');

import 'MyLegacyClass' with { document: myiframe.contentWindow.document };

var frameBody = myiframe.contentWindow.document.body

new MyLegacyClass().toElement().inject(framebody); // Doesn't work without proper IoC

MyLegacyClass.js

var MyLegacyClass = new Class({

    initialize: function(){
        this.element = new Element('div');
    },

    toElement. function(){
        return this.element;
    }

});