Share Object.assign polyfill between UMD builds

[gaearon]

This wins back 0.3k post min/gzip which we lost due to adding license headers 😛 We can later share more utilities like this, since currently it's suboptimal that some fbjs things are duplicated. This only affects UMD builds.

How I verified it works:

• Fixtures work (but they were broken between changing the bundling logic and adding an export)
• The combined size has decreased
• The diff makes sense (https://gist.github.com/gaearon/75431ad02d69a08e95aef6fba2645244), most of it is renaming index to assign though (since Rollup gives it a different name now)

It only affects UMD builds and doesn’t affect any other builds.
This is not global shared state, it's just manual deduplication.

[sophiebits]

haha, ok