Call Daemon.check_entry_point ASAP
Call Daemon.check_entry_point ASAP

Summary: `Daemon.spawn` sets an environment variable and then calls `flow`, expecting to hit a call to `Daemon.check_entry_point ()` which will hijack execution to call into the "entry point" specified in the environment variable.

We weren't calling `Daemon.check_entry_point ()`. Hack calls it in `serverMain.ml`, but since they have separate client and server binaries, `serverMain.ml` gets hit. our server codepath doesn't get hit unless you call `flow server`; everything else is a client command that execs `flow server`, so we have to do this right upfront.
Closes https://github.com/facebook/flow/pull/758

Reviewed By: @mroch, @gabelevi

Differential Revision: D2393928

Pulled By: @bhosmer

JonathanUsername/flow,jeffmo/flow,Fresheyeball/flow,mroch/flow,Wanderfalke/flow,dv/flow,shiftcars/flow,Drooids/flow,samwgoldman/flow,JonathanUsername/flow,JonathanUsername/flow,jeffmo/flow,jeffutter/flow,gabro/flow,ylu1317/flow,darioajr/flow,KevinTCoughlin/flow,siddhartharay007/flow,danvk/flow,jeffutter/flow,AgentME/flow,OCamlPro/flow,dv/flow,claudiopro/flow,ylu1317/flow,mroch/flow,AprilArcus/flow,mroch/flow,ylu1317/flow,graue/flow,popham/flow,AprilArcus/flow,graue/flow,AprilArcus/flow,cesarandreu/flow,jeffutter/flow,jdelStrother/flow,dv/flow,jamesgpearce/flow,Fresheyeball/flow,gabelevi/flow,fletcherw/flow,popham/flow,Fresheyeball/flow,facebook/flow,jgrund/flow,jgrund/flow,KevinTCoughlin/flow,cesarandreu/flow,facebook/flow,KevinTCoughlin/flow,jeffmo/flow,graue/flow,ForbesLindesay/flow,paulshen/flow,graue/flow,paulshen/flow,AgentME/flow,nmote/flow,d14b3l/flow,JohnyDays/flow,asivitz/flow,jamesgpearce/flow,TiddoLangerak/flow,AgentME/flow,mroch/flow,asivitz/flow,danvk/flow,gabro/flow,gabelevi/flow,siddhartharay007/flow,jamesgpearce/flow,Eyyub/flow,mcanthony/flow-type,fletcherw/flow,jamesgpearce/flow,ForbesLindesay/flow,asivitz/flow,JonathanUsername/flow,paulshen/flow,gabelevi/flow,claudiopro/flow,danvk/flow,samwgoldman/flow,ylu1317/flow,OCamlPro/flow,popham/flow,asivitz/flow,JohnyDays/flow,fletcherw/flow,Eyyub/flow,AprilArcus/flow,claudiopro/flow,JonathanUsername/flow,nmote/flow,Eyyub/flow,Fresheyeball/flow,d14b3l/flow,shiftcars/flow,claudiopro/flow,gabelevi/flow,popham/flow,MichaelDeBoey/flow,chenglou/flow,d14b3l/flow,chenglou/flow,gabro/flow,gabelevi/flow,samwgoldman/flow,TiddoLangerak/flow,facebook/flow,ForbesLindesay/flow,chenglou/flow,mroch/flow,MichaelDeBoey/flow,Eyyub/flow,Wanderfalke/flow,popham/flow,fletcherw/flow,Drooids/flow,mcanthony/flow-type,MichaelDeBoey/flow,TiddoLangerak/flow,siddhartharay007/flow,mroch/flow,popham/flow,dv/flow,Wanderfalke/flow,gabro/flow,jgrund/flow,d14b3l/flow,jdelStrother/flow,claudiopro/flow,jgrund/flow,graue/flow,fletcherw/flow,nmote/flow,TiddoLangerak/flow,gabro/flow,jgrund/flow,ForbesLindesay/flow,jeffmo/flow,TiddoLangerak/flow,samwgoldman/flow,mcanthony/flow-type,chenglou/flow,cesarandreu/flow,jamesgpearce/flow,MichaelDeBoey/flow,fletcherw/flow,d14b3l/flow,ForbesLindesay/flow,Drooids/flow,JohnyDays/flow,nmote/flow,darioajr/flow,mcanthony/flow-type,shiftcars/flow,JonathanUsername/flow,jdelStrother/flow,JohnyDays/flow,siddhartharay007/flow,graue/flow,AgentME/flow,KevinTCoughlin/flow,facebook/flow,ForbesLindesay/flow,AprilArcus/flow,gabelevi/flow,MichaelDeBoey/flow,nmote/flow,jeffmo/flow,AprilArcus/flow,danvk/flow,ylu1317/flow,facebook/flow,shiftcars/flow,samwgoldman/flow,popham/flow,OCamlPro/flow,nmote/flow,darioajr/flow,jgrund/flow,jeffmo/flow,jeffutter/flow,gabro/flow,jdelStrother/flow,jdelStrother/flow,AgentME/flow,jamesgpearce/flow,shiftcars/flow,d14b3l/flow,facebook/flow,samwgoldman/flow,jamesgpearce/flow,samwgoldman/flow,mroch/flow,jdelStrother/flow,chenglou/flow,chenglou/flow,OCamlPro/flow,facebook/flow,Drooids/flow,Wanderfalke/flow,gabelevi/flow,nmote/flow,paulshen/flow,shiftcars/flow,darioajr/flow,JonathanUsername/flow,cesarandreu/flow