Module render performance tweak - #823

/cc @Rich-Harris Do you think it's safe?

This was not safe, and started breaking my build.

Previously with v0.34.3 my format: 'es' build worked correctly, then it started failing after this change. It was changing React.createElement to React__default.createElement but never defining React__default.

An example of the issue is here: https://travis-ci.org/yahoo/react-intl/jobs/150944243#L1425

I kept installing all the recent versions until things broke, and narrowed it down to this change.

@ericf Thank you for report. Will be fixed ASAP.

@ericf Can you produce some minimal test case? Can't figure out how to build your repo on windows.

@TrySound I figured out the issue, and it makes sense why this change would cause it. I am calling rollup() once, but calling bundle() twice. I created this script before the targets feature was added: https://github.com/yahoo/react-intl/blob/master/scripts/build-lib.js#L31-L41

I will update my script to be more correct to fix this.

@ericf - well? did the merge break your code or all is good?

@yairEO the change in this PR was undone in 1d7c853