Add static injection for feature flags

[gaearon]

Related to discussion in #11260.

This does two things:

• Removes ReactDOMFeatureFlags as a separate entity, instead opting for unified feature flag list across the whole codebase in ReactFeatureFlags.

• Adds a mechanism to specify build-time overrides for the whole ReactFeatureFlags file for a particular bundle.

For example, let’s say we have a flag called includePersistenceImplementation, and react-native-cs wants to flip it to true. To introduce it, we would need to:

1. Add it to ReactFeatureFlags.js:

export type FeatureFlags = {|
  enableAsyncSubtreeAPI: boolean,
  enableAsyncSchedulingByDefaultInReactDOM: boolean,
+  includePersistenceImplementation: boolean
|};

var ReactFeatureFlags: FeatureFlags = {
  enableAsyncSubtreeAPI: true,
  enableAsyncSchedulingByDefaultInReactDOM: false,
+ includePersistenceImplementation: false,
};

2. Add new ReactNativeCSFeatureFlags.js:

import type {FeatureFlags} from 'ReactFeatureFlags';

const ReactNativeCSFeatureFlags: FeatureFlags = {
  enableAsyncSubtreeAPI: true,
  enableAsyncSchedulingByDefaultInReactDOM: false,
  includePersistenceImplementation: true, // <---- the important part
};

module.exports = ReactNativeCSFeatureFlags;

Note how it has to include all flags since we’re fully forking the file. It might seem a bit annoying but it’s simpler than figuring out how to merge them both at compile, Flow, and test time. Exact Flow types here guard us against mistakes.

3. Use it in the build config:

    name: 'react-native-cs-renderer',
+   featureFlags: 'src/renderers/native-cs/ReactNativeCSFeatureFlags',
    paths: [
      'src/renderers/native-cs/**/*.js',
      'src/renderers/shared/**/*.js',

We can remove this step and make it conventional after #11252.

4. Use it in tests when necessary:

jest.mock('ReactFeatureFlags', () => require('ReactNativeCSFeatureFlags'));

This should be enough to enable DCE for cases like #11260 because GCC (which we use for react-dom) is smart enough to read static boolean flags.

Potential pitfalls of this approach:

• Might not be obvious ReactFeatureFlags needs to be mocked in tests.
• Might not be obvious flags need to be specified in all overrides, rather than merged.

Potential future work:

• Remove the configuration bit and locate per-package FeatureFlags override automatically.
• Generalize this as a static injection system for overriding any modules.
• Maybe split out into ReactBuildFlags and ReactRuntimeFlags for extra clarity?

[trueadm]

LGTM

[bvaughn]

Remove the configuration bit and locate per-package FeatureFlags override automatically.

Using convention to auto-locate src/renderers/*/FeatureFlags.js (if one exists) sounds nice as a potential follow-up.