Gregory Benner 0c4845f2ff [flow] typecheck getParam to actually check that key and fallback are valid (#5074)
* Update flow type definition for getParam to actually check key and
fallback are valid

* add changelog entry for `getParam` flow type changes

* Return "any" in `getParam` if params are not defined in the navigation state

* use void in place of any
2018-10-19 09:57:34 -07:00
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