Allow reading default feature flags from bundle tests

[gaearon]

I want to run some tests both in Fire and normal modes (#13628).

But for that I need tests to switch on ReactFeatureFlags and determine the expected behavior based on that. And this doesn't work in bundle tests so I'd have to make them internal.

I don't want to make those tests internal. So instead I'm making a readonly version of ReactFeatureFlags available to bundle tests. That's sufficient to know in which mode we're running. If you attempt to write to it (as some our tests which need to stay internal do), this will throw.

I could have used something like process.env for this instead. But it felt more confusing to me to have different ways to read a feature flag.

[sizebot]

Details of bundled changes.

Comparing: 7204b63...44118e3

schedule

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
schedule.development.js	n/a	n/a	0 B	19.17 KB	0 B	5.74 KB	UMD_DEV
schedule.production.min.js	n/a	n/a	0 B	3.16 KB	0 B	1.53 KB	UMD_PROD
schedule-tracking.development.js	-4.5%	-8.4%	10.64 KB	10.16 KB	2.51 KB	2.3 KB	NODE_DEV

Generated by 🚫 dangerJS