Summary:
Before:
```
flipper-server/src/startBaseServer.tsx(222,30): error TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
Type 'undefined' is not assignable to type 'string'.
flipper-ui-browser/src/flipperServerConnection.tsx(25,9): error TS2322: Type 'Timeout' is not assignable to type 'number'.
```
Reviewed By: nikoant
Differential Revision: D36100188
fbshipit-source-id: 1913a43109a0e068394a188d362f6a9e473e7904
3032736294