Use mergeFast in a hotspot

[rigdern]

In profiling our app, we found that the usage
of merge in Text.js was showing up as a
hotspot. We've replaced this usage of merge
with mergeFast.

Test plan (required)

This change is used in my team's app.

Adam Comella
Microsoft Corp.

[ghost]

By analyzing the blame information on this pull request, we identified @JoelMarcey and @MattFoley to be potential reviewers.

[MattFoley]

This is the first time I've been poked by Github bot, but this change looks good to me.

The only suggestion I would have is to use let or even better const, rather than var, but I don't know RN's policy on using ES6 syntax in the main repo. (They may want to not for now, as it takes some specific babel plugins.)

[ide]

merge(a, b) and mergeFast(a, b) aren't guaranteed to be the same for all (a, b), which is concerning since merge(a, b) is run in__DEV__ but not production. Keeping dev and production the same as much as possible is one of the best ways to head off bugs.

It would be better either to use mergeFast in all environments or write a new helper function like mergeOwnPropertiesFast with the same semantics as merge by doing hasOwnProperty checks or iterating over Object.keys, whichever's faster.

[rigdern]

@ide you make a good point about dev vs production. I updated my PR to use mergeFast in all environments.

[ghost]

@rigdern updated the pull request - view changes

[ide]

@facebook-github-bot shipit

[ghost]

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