I'm not using Bluebird but similarly to this thread I'm getting the same warning errors of unhanded rejection when actually expecting a rejection.

Here's a repo to reproduce: https://github.com/laggingreflex/chai-as-promised-reject-test

It was mentioned chai-as-promised is indeed handling the errors (here) but I think they're not being handled soon enough. Putting a console.log there shows that it reaches that code only after the warning has been displayed:

(node:5580) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: rejected
  promise
(node:5580) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)
>>> assertIfNegated about to be called... <<<
    √ should be rejected


  1 passing (48ms)