code: @Nullable
    static FeatureSet getFeatureSetOfScript(Node scriptNode) {
      checkState(scriptNode.isScript(), scriptNode);
      return (FeatureSet) scriptNode.getProp(Node.FEATURE_SET);
    }
docs: Returns a script node's FeatureSet, which is set at parse-time. This may not be up-to-date
  as passes can add/remove features from a script node's descendants.
  
  <p>The feature set will be null if the script node was created artificially or if the parser
  didn't detect any interesting features.