code: private static ImmutableSet<String> getFunctionParameterSet(Node fnNode) {
      ImmutableSet.Builder<String> builder = ImmutableSet.builder();
      for (Node n : NodeUtil.getFunctionParameters(fnNode).children()) {
        if (n.isRest()){
          builder.add(REST_MARKER);
        } else if (n.isDefaultValue() || n.isObjectPattern() || n.isArrayPattern()) {
          throw new IllegalStateException("Not supported: " + n);
        } else {
          builder.add(n.getString());
        }
      }
      return builder.build();
    }
docs: Get a set of function parameter names.