styles: replace indexOf() with includes()

[ghost]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

All the subsystems and tests using indexOf().

Description of change

nodej.s version is 8 / 9 in .travis.yml , so I make some changes to make code more readable by using ES6 features. The rules that I used:

• replace indexOf() !== -1 with includes()
• replace indexOf() >= 0 with includes()
• replace indexOf() > -1 with includes()
• replace indexOf() === -1 with !includes()

[codecov-io]

Codecov Report

Merging #148 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #148   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          17     17           
  Lines         881    881           
=====================================
  Hits          881    881

Impacted Files	Coverage Δ
lib/loader/mixin/plugin.js	100% <100%> (ø)	⬆️
lib/utils/sequencify.js	100% <100%> (ø)	⬆️
lib/loader/egg_loader.js	100% <100%> (ø)	⬆️
lib/utils/router.js	100% <100%> (ø)	⬆️

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[fengmk2]

Thanks!