code: private void removeUnneededPolyfills(Node parent, Node runtimeEnd) {
      Node node = parent.getFirstChild();
      while (node != null && node != runtimeEnd) {
        Node next = node.getNext();
        if (NodeUtil.isExprCall(node)) {
          Node call = node.getFirstChild();
          Node name = call.getFirstChild();
          if (name.matchesQualifiedName("$jscomp.polyfill")) {
            FeatureSet nativeVersion =
                FeatureSet.valueOf(name.getNext().getNext().getNext().getString());
            if (languageOutIsAtLeast(nativeVersion)) {
              NodeUtil.removeChild(parent, node);
            }
          }
        }
        node = next;
      }
    }
docs: that already contains the library) is the same or lower than languageOut.