code: function safelyShadowsUndefined(variable) {
      return variable.name === "undefined" &&
          variable.references.every(ref => !ref.isWrite()) &&
          variable.defs.every(def => def.node.type === "VariableDeclarator" && def.node.init === null);
  }
docs: Determines if a variable safely shadows undefined.
  This is the case when a variable named `undefined` is never assigned to a value (i.e. it always shares the same value
  as the global).
  @param {eslintScope.Variable} variable The variable to check
  @returns {boolean} true if this variable safely shadows `undefined`