Flow: Fix generating arrow functions with param

[danharper]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	no
Tests added/pass?	yes
Fixed tickets	#4503, #4211
License	MIT
Doc PR	-

Fixes:

• transform-flow-comments single arrow param would not print type (babel-plugin-transform-flow-comments does not preserve types for single parameter lambda functions #4503)

# const x = (foo: string) => {};
- const x = foo => {};
+ const x = (foo /*: string*/) => {};

• transform-flow-comments single arrow param would not print optional (babel-plugin-transform-flow-comments failing for ? optional/maybe (T7249) #4211)

# const x = (foo?) => {};
- const x = foo => {};
+ const x = (foo /*:: ?*/) => {};

I also found just using syntax-flow wouldn't print out types/optionals for arrow functions with a single param. e.g. all of these:

const bar1 = (x: number): string => {};
const bar2 = (x?) => {};
const bar3 = (x?: string) => {};
const bar4 = x => {};
const bar5 = (x): string => {};
const bar6 = (x: number) => {};
const bar7 = <T>(x) => {};

(only when they have a single param) output:

const bar1 = x => {};
const bar2 = x => {};
const bar3 = x => {};
const bar4 = x => {};
const bar5 = x => {};
const bar6 = x => {};
const bar7 = x => {};

The output now correctly matches the input.

[codecov-io]

Current coverage is 88.32% (diff: 100%)

Merging #4504 into master will increase coverage by <.01%

@@             master      #4504   diff @@
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  Lines         13609      13612     +3   
  Methods        1426       1427     +1   
  Messages          0          0          
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+ Hits          12020      12023     +3   
  Misses         1589       1589          
  Partials          0          0          

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[hzoo]

Might be confusing to read to some but makes sense.

[danharper]

ah yeah, idk why I didn't assign it immediately. I've pushed a change for that (and extracted the type checking into a function to be explicit).

[hzoo]

Awesome

[danez]

👍 Thanks alot

[hzoo]

🎉 great stuff