[META] conditional babel presets when bundling JS

[dulmandakh]

In version 0.56, iOS 9 is now the minimum required version(f50df4f). And on iOS 9, Webkit and JSC partially supports ES6 (https://webkit.org/blog/4054/es6-in-webkit/).

So it might be better to not transpile natively supported ES6 features to have native performance. Especially for release builds.

Relevant blog posts:

• https://webkit.org/blog/4054/es6-in-webkit/
• https://webkit.org/blog/6756/es6-feature-complete/
• https://webkit.org/blog/7536/jsc-loves-es6/

[dulmandakh]

RN might land newer JSC on Android, and support many new JS features #19737

[react-native-bot]

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