code: function findPropsWithFunctionExpression (propsNode, propname) {
        return propsNode
          .filter(prop => prop.type === 'FunctionExpression')
          .filter(prop => {
            return hasUsingProp(prop, propname)
          })
      }
docs: Finds all props that don't have a default value set
  @param {Property} propsNode - Vue component's "props" node
  @return {boolean}