[PR] Bump acorn from 5.7.3 to 5.7.4
[PR] Bump acorn from 5.7.3 to 5.7.4

Summary:
Bumps [acorn](https://github.com/acornjs/acorn) from 5.7.3 to 5.7.4.
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/acornjs/acorn/commit/6370e90067552022710190319cbbbd8c43001957"><code>6370e90</code></a> Mark version 5.7.4</li>
<li><a href="https://github.com/acornjs/acorn/commit/fbc15b1344f6dfb992f67b4bbf1357436247c8a0"><code>fbc15b1</code></a> More rigorously check surrogate pairs in regexp validator</li>
<li>See full diff in <a href="https://github.com/acornjs/acorn/compare/5.7.3...5.7.4">compare view</a></li>
</ul>
</details>
<br />

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</details>
Pull Request resolved: https://github.com/facebook/flow/pull/8323

Reviewed By: panagosg7

Differential Revision: D21065118

Pulled By: mroch

fbshipit-source-id: cd42be889678b99f1c930f8726721166e95aa793

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