t newInstance = null;
    function HTMLTemplateElementExtension() { return newInstance; }
    HTMLTemplateElementExtension.prototype = Object.create(HTMLTemplateElement.prototype, {
      constructor: {
        value: HTMLTemplateElementExtension,
        writable: true
      }
    });
    const DataTemplate = Polymer.PropertyEffects(HTMLTemplateElementExtension);
    const MutableDataTemplate = Polymer.MutableData(DataTemplate);

    // Applies a DataTemplate subclass to a <template> instance
    function 