Add test for successful typecheck from dependent module

[paulirish]

A test for https://github.com/GoogleChrome/chrome-launcher/issues/81

see also https://github.com/GoogleChrome/lighthouse/pull/4192#issuecomment-356150346

i was thinking a more functional test. e.g. some module that require()s chrome-launcher successfully typechecks with TS.

[paulirish]

I attempted this in a branch: https://github.com/GoogleChrome/chrome-launcher/compare/fixtstypingsattempt

however i can't get this 'Could not find a declaration file' error. :(