Babel 7 support and additional enhancements

[novemberborn]

Fixes #1598. Switches AVA's Babel implementation to use Babel 7. This applies to test and helper file compilation.

Adds a compileEnhancements option which can be set to false to disable Power Assert and our t.throws() helper.

Changes the Babel configuration. If you had this before:

"ava": {
  "babel": {
    "plugins": []
  }
}

You'll now need:

"ava": {
  "babel": {
    "testOptions": {
      "plugins": []
    }
  }
}

ava.babel.testOptions.babelrc now defaults to true. You can disable our stage-4 preset by adding ["ava/stage-4", false] to ava.babel.testOptions.presets. Set ava.babel to false to disable AVA's test file compilation, whilst still compiling enhancements. If compileEnhancements is also set to false then Babel is skipped completely.

Fixes #1225, #1488 and #1556.

[haltcase]

All seems to work so far! Was up and running using new @babel plugins in tests. .babelrc.js support is something I'm looking forward to.

[danny-andrews]

I tried to give this a whirl. I'm getting Cannot find module 'hullabaloo-config-manager'

[novemberborn]

@danny-andrews are you installing using npm 5.6? Currently these are Git dependencies, and hullabaloo-config-manager has a build step, so earlier npm versions might not handle them particularly well.

[haltcase]

@novemberborn would it be premature to publish these at @next tags? That'd make it more accessible for testing so is that the plan now at some point?

[novemberborn]

@citycide I think so, yes. There's five dependencies that need updating, one of which we don't have control over. I haven't seen a clear timeline for when Babel 7 goes stable, and there've been significant changes over the past few weeks alone. For now let's stick to installing from this GitHub branch.

[danny-andrews]

@novemberborn I used yarn. I'll just hold off until things progress. Found a workaround to get pipeline-operator to work without upgrading to babel 7. :) (I'd happy to test in out once it's in beta or whatever, though.)

[novemberborn]

@citycide I'm hopeful that .babelrc.js files work now. Either through the babelrc option or by extending from JS files. Note that AVA does not yet take dependency changes into account if you use require() inside those JS configuration files.

There's a test failure in Node.js 4 in novemberborn/hullabaloo-config-manager#25 which might just be the test itself, not the code, but I don't have the time to figure that out right now.

[novemberborn]

Also this PR now selects the test environment if BABEL_ENV and NODE_ENV are not set, similar to how we set NODE_ENV in the workers.

[haltcase]

@novemberborn it works 🙌

tested it on:
node v8.9.3, v6.12.2
npm v5.6.0, v3.10.10
babel v7.0.0-beta.34 w/ @babel/preset-stage-0 ( used pipeline and nullish coalescer in test 😋 )

For others who want to test, yarn didn't install the hullabaloo-config-manager dependency correctly. Once I used npm it worked fine.

[novemberborn]

Please see the updated PR description.

I'll publish beta releases of our various dependencies and then merge this into master.