Fix flow plugin when flow+arrow+spread used together

[danez]

Fix for

( ...props: SomeType ) : ?ReturnType => ( 3 );

The fix creates a new method on the parser called parseArrow.
This new function by default only checks if current position matches an
arrow. If it does returns the node otherwise undefined.
The flow plugin can then extend this function and correctly parse the typeAnnotation
and add it to the node.

With this change, in the flow plugin there is no need anymore to extend
parseParenAndDistinguishExpression and the arrow handling in parseParenItem
could also be removed, because it is all handled now in parseArrow.

Some existing tests were failing, because extra->parentesized is now missing,
but this should be correct as it is now inline with the parsing without flow annotations. No extra
is added for arrow function without type annotations.

Also, in the expression-parser this.next() was replaced by a more specific
this.expect(tt.parenL) inside parseParenAndDistinguishExpression.

And I reverted the fix from babel/babel#3305 as it is not necessary anymore.

Fixes babel/T2612

[loganfsmyth]

This was auto-closed because this repo has been updated to include the Git history from the main Babel repo. I can't speak to the validity of this PR, but it appears you'll need to rebase and open a new one.

The new force-pused master branch should be identical to the old one, so you should be able to use rebase --onto or cherry-pick to move your commit over.