Document how to use babel-polyfill to polyfill the test environment

[novemberborn]

AVA does not load polyfills when running tests. This means features like ['foo', 'bar'].includes('foo') are not available. We should update the ES2015 docs to point this out, and add a recipe explaining how to use babel-polyfill with AVA.

There should be a caveat to the latter that users shouldn't load babel-polyfill unless they also use it in their applications, since the polyfill will apply to both test and source files.

See #263 for background.

[ahmednuaman]

Isn't it just about having this in the package.json:

"ava": {
  "require": [
    "babel-register",
    "babel-polyfill"
  ]
},

[florianb]

@novemberborn: following Sindre's call to the weapons - i'd like to take this issue if it is still relevant and nobody else is working on it!

[novemberborn]

@florianb that's awesome, go for it!

[florianb]

@novemberborn - it seems like this issue is invalid, since Array.include() is being part of the standard from ES2016+. Ava seems to support ES2017 out of the box, there is no ES2015-sepecific section in the docs anymore.

However, i might be wrong or i also could still add an ES2015-section to the docs. I'd appreciate any hint. 😄

[novemberborn]

Ava seems to support ES2017 out of the box, there is no ES2015-sepecific section in the docs anymore.

ES2017 includes ES2015 😉

https://github.com/avajs/ava/blob/master/docs/recipes/babelrc.md has further details beyond what is in the main README. The issue is that we can only transpile syntax, we don't modify built-ins. It's documented in https://github.com/avajs/babel-preset-stage-4, which is linked to, but we could spell it out better in at least the babelrc recipe.

This is why we should still document babel-polyfill.

[florianb]

Aye - i then will add a proposal for the babelrc. Thanks for clarification.

[florianb]

@novemberborn: Change drafted, i would appreciate any feedback and since this is my first contribution i guess there is room for improvement. Thank you very much in advance.