code: function ensurePrototypeTraversal(prototype) {
      if (!Object.__proto__) {
        var ancestor = Object.getPrototypeOf(prototype);
        prototype.__proto__ = ancestor;
        if (isBase(ancestor)) {
          ancestor.__proto__ = Object.getPrototypeOf(ancestor);
        }
      }
    }
docs: On platforms that do not support __proto__ (versions of IE), the prototype chain of a custom element is simulated via installation of __proto__. Although custom elements manages this, we install it here so it's available during desugaring.