fs.mkdir definition missing

[ttsui]

Summary
When using fs.mkdir with memory-fs it doesn't work because its not mapped.

Test plan
Can test by attempting a call to fs.mkdir(file_path)

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[codecov-io]

Codecov Report

Merging #456 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #456   +/-   ##
=======================================
  Coverage   84.45%   84.45%           
=======================================
  Files         173      173           
  Lines        5763     5763           
  Branches      948      948           
=======================================
  Hits         4867     4867           
  Misses        794      794           
  Partials      102      102

Impacted Files	Coverage Δ
packages/metro-memory-fs/src/index.js	90.74% <ø> (ø)	⬆️

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[cpojer]

Thanks!

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@cpojer merged this pull request in a5b6bf8.