use `void 0` to replace 'undefined'

[newPromise]

use void 0 to replace 'undefined' for prevent rewrite.

[jonschlinkert]

I'll need some help understanding this. Please describe the problem first (be specific).

[newPromise]

because when under ES5 ， especially in IE browser, identifier undefined can be rewrited, like this example:

var undefined = "hello";
console.log(undefined === "hello"); // true

under Chrome browser or use strict mode no question;

because you use strict mode, so it's well;

so sorry~, my English is not well...

[jonschlinkert]

want to do a pr?

[anonyco]

Please do not do this. It makes the code harder to read and is already done by decent minifiers like closure compiler. If you want smaller code, run the Javascript through a minifier instead of trying to compact the source code.