Enable flowComments for babel-flow

[sosukesuzuki]

#9062 (comment)

• I’ve added tests to confirm my change works.
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[fisker]

What about babel? It support flow, We'll have to deal with it too.

I hate this , in one hand we are trying to removeflow from babel, in another hand we have to add more stuff for it.

[thorn0]

What about babel? It support flow

My opinion is that it's fine to ignore Flow comments in babel. Its support for Flow isn't reliable anyway.

[fisker]

We already know it's broken, ignoring the problems seems not good.

How about redirect to babel-flow too, if /*:: found #9071

[thorn0]

These comments are a little-used and little-known feature of Flow. If someone goes to the trouble of using them, it shouldn't be difficult for this person to add @flow or switch parsers in Prettier.

[thorn0]

This PR misses this part from #9062 (comment): "and unify the code for flow and babel-flow".
Code like the hasFlowShorthandAnnotationComment and hasFlowAnnotationComment functions was added for Babel only because back then Babel didn't have the flowComments plugin. Hopefully it should be safe to remove them now.

[thorn0]

Superseded by #13687