Ability to use ES6: rollup+babel transpilation

[drzraf]

This change gives the opportunity to use ES6 for src/flow.js

• karma.conf.js : Test the transpiled bundle instead of ES6 code.
• babel: use bable-env + browserlist to transpile for a wider range of browsers.
• For that purpose, switch to rollup with way handier and more modern than browserify.
• In order to keep the toolchain backward-compatible (and reduce the PR surface), use grunt:exec to wrap rollup execution.

Also update to karma-coverage v2 needed for async support as discussed and anticipated in #304.

[AidasK]

You are doing a huge upgrade and that's great. Would you like to be added as a maintainer of this project?

[drzraf]

As long as someone keeps reviewing PR, I've no problem with this but I can't guarantee long-term involvement.

Still, about that specific PR, I don't understand why the CI fails, is this something you could fix to get this green?

[AidasK]

It looks like it can't generate karma coverage report because coverage plugin is missing. Though all the tests pass.
You can see the log here: https://travis-ci.org/github/flowjs/flow.js/jobs/723863503

[drzraf]

I guess the plugin is not loaded (anymore) because node can't parse node_modules/karma-sauce-launcher. Updating the version of node in .travis.yml may do it?

[AidasK]

https://travis-ci.org/github/flowjs/flow.js/jobs/724987732
now we are getting different error

/home/travis/.nvm/versions/node/v14.9.0/lib/node_modules/codeclimate-test-reporter/formatter.js:33
      throw parseError;
      ^
Unknown input coverage format
(Use `node --trace-uncaught ...` to show where the exception was thrown)
The command "$TRAVIS_BUILD_DIR/travis.sh" exited with 1.

It looks like coverage was created, but with different format or empty file.

[AidasK]

can you test if grunt karma:coverage generates a valid coverage report? e.g. it should create coverage/*/lcov.info files

[drzraf]

Could you set your CC_TEST_REPORTER_ID if different from CODECLIMATE_REPO_TOKEN?
Note: These variables (like SAUCE_ACCESS_KEY) should probably be secret and stored inside GitHub configuration.
You may also want to remove some configuration from the browsers sauce test against.

It's not doing coverage reporting (0-size file). But I don't understand which tool is actually in charge of code instrumentation?

[AidasK]

Great job, it works! I have added you to the maintainers of this project. Once finished, your branch will be released as v3.0.0 flowjs since it supports promises. Please don't forget documentation too 🥇

[drzraf]

Does not work. lcov.info is 0 bytes.

[drzraf]

code-coverage fixed.
Should I let you squash-merge on -master? Or on a v3 branch if you prefer such solution the time being?

[AidasK]

Yes please, squash it and push it to v3 branch. Feel free to do it yourself, you have all the permissions you need. We need some time to release it to master once it's tested.

[drzraf]

Note: The update to karma-coverage v2 caused start --single-run=false to stop functioning.
(karma-runner/karma-sauce-launcher#217)
Will fix this in a subsequent commit.