Fix stdin with async-to-promises

[fisker]

Another solution for #6891, I'm Ok with any of them

#6894
#6893

also lacal tested

@lydell

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file size

npm notice package: prettier@1.20.0-dev
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• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/pr-XXXX.md file following changelog_unreleased/TEMPLATE.md.
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[lydell]

As far as I understand, we won’t need this in 2.0 so I’m closing. Let me know if I’ve misunderstood anything.

Closing in favor of #6894.