Refactor Scheduler.js

[alexnoz]

es6+ refactoring of Scheduler.js

[aweary]

Thanks for taking the time to do this @alexnoz! It was intentionally written this way. See #13582 for some context.

[sizebot]

Details of bundled changes.

Comparing: 4d17c3f...da413e3

scheduler

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
scheduler.development.js	n/a	n/a	0 B	19.17 KB	0 B	5.74 KB	UMD_DEV
scheduler.production.min.js	n/a	n/a	0 B	3.16 KB	0 B	1.53 KB	UMD_PROD
scheduler.development.js	+1.1%	+0.9%	21.35 KB	21.59 KB	5.75 KB	5.8 KB	NODE_DEV
scheduler.production.min.js	🔺+1.4%	🔺+0.7%	4.76 KB	4.83 KB	1.85 KB	1.86 KB	NODE_PROD
Scheduler-dev.js	+1.5%	+1.0%	21.62 KB	21.94 KB	5.8 KB	5.86 KB	FB_WWW_DEV
Scheduler-prod.js	🔺+1.1%	🔺+0.7%	13.3 KB	13.45 KB	2.87 KB	2.89 KB	FB_WWW_PROD

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