code: private static Node returnedExpression(Node fn) {
      Node expectedBlock = NodeUtil.getFunctionBody(fn);
      if (!expectedBlock.hasOneChild()) {
        return null;
      }
  
      Node expectedReturn = expectedBlock.getFirstChild();
      if (!expectedReturn.isReturn()) {
        return null;
      }
  
      if (!expectedReturn.hasOneChild()) {
        return null;
      }
  
      return expectedReturn.getOnlyChild();
    }
docs: Return the node that represents the expression returned
  by the method, given a FUNCTION node.