src/language-js/print/function-parameters.js/getReturnTypeNode
function getReturnTypeNode(functionNode) {
  let returnTypeNode;
  if (functionNode.returnType) {
    returnTypeNode = functionNode.returnType;
    if (returnTypeNode.typeAnnotation) {
      returnTypeNode = returnTypeNode.typeAnnotation;
    }
  } else if (functionNode.typeAnnotation) {
    returnTypeNode = functionNode.typeAnnotation;
  }
  return returnTypeNode;
}