Remove Array.prototype.includes and Array.prototype.findIndex (fix #608)

[honeroku]

TODO

• make test if possible, or add eslint rule.
  • Use of Array.prototype.includes breaks unit tests in browsers that do not support this. #608 (comment)
• Run some unit test using @vue/test-utils

[lusarz]

Great finding!

[honeroku]

@eddyerburgh I investigated eslint-plugin-compat with browserslist: ["last 1 versions", "Explorer >= 9"] according to vue supported browsers but had no luck to pick up those includes and findIndex on dev branch.
And I have no other idea to detect browser compatibility.

Ashamed of myself but would you mind merging this PR without systematic testimony?

[eddyerburgh]

Thanks :)

[honeroku]

Thank you!!

[eddyerburgh]

The eslint-compat-plugin doesn't catch lots of unsupported features right now—amilajack/eslint-plugin-compat#72 (comment)