Adding chai-as-promised to an Ember app

I'm finding myself in an Ember-based app and are having a little trouble understanding how I should add the chai-as-promised helper library to it. I'm running this version:

$ ember --version
version: 2.4.2
node: 5.8.0
os: darwin x64

I started by installing via npm i chai-as-promised --save-dev. The library was then importable via Node. Then I have tried adding it to the ember-cli-build.js file using two different approaches:

As a file via .import(), after creating the EmberApp:

  module.exports = function(defaults) {
    var app = new EmberApp([...]);
    app.import('./node_modules/chai-as-promised/lib/chai-as-promised.js');

Via EmberApp.toTree() to chai-as-promised's top directory:

return app.toTree('./node_modules/ember-cli-blueprint-test-helpers/');

And descending into the lib/ subdirectory of chai-as-promised:

return app.toTree('./node_modules/chai-as-promised/lib');

I also tried installing via Bower and changing the above node_modules/ based paths to bower_components ones, but still with the same result.

Am I importing it wrong? Or is there somewhere else I should import?