[metro] add $FlowExpectedError annotation for optional visualizer dep

[Salakar]

This adds a $FlowExpectedError comment annotation for the optional metro-visualizer dependency, resolves the following issue:

Summary

Test plan

[codecov-io]

Codecov Report

Merging #278 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #278   +/-   ##
=======================================
  Coverage   85.75%   85.75%           
=======================================
  Files         144      144           
  Lines        4548     4548           
  Branches      712      712           
=======================================
  Hits         3900     3900           
  Misses        580      580           
  Partials       68       68

Impacted Files	Coverage Δ
packages/metro/src/index.js	74.69% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a62c79...044d1e4. Read the comment docs.

[rafeca]

Thanks! it's strange that this is failing in the React Native repo but not in the Metro repo...

[facebook-github-bot]

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

[Salakar]

It is strange, there was a dev dependency but removing that locally made no difference, I had to physically delete the metro-visuallizer directory in packages/ locally to be able to reproduce the error - so must be a yarn workspaces thing, one to note for future I guess 🙈