add --check flag to `flow ast` which returns errors but no AST
add --check flag to `flow ast` which returns errors but no AST

Summary: sometimes you just want to see whether a file parses but don't care about its AST

Reviewed By: samwgoldman

Differential Revision: D7835876

fbshipit-source-id: 23744b8190d55029f1d085c9e9d41a8fed967e5a

gabelevi/flow,facebook/flow,AgentME/flow,popham/flow,samwgoldman/flow,popham/flow,samwgoldman/flow,samwgoldman/flow,popham/flow,facebook/flow,nmote/flow,jamesgpearce/flow,gabelevi/flow,facebook/flow,JonathanUsername/flow,JonathanUsername/flow,JonathanUsername/flow,mroch/flow,mroch/flow,AgentME/flow,jamesgpearce/flow,facebook/flow,samwgoldman/flow,JonathanUsername/flow,jamesgpearce/flow,mroch/flow,nmote/flow,facebook/flow,nmote/flow,gabelevi/flow,AgentME/flow,mroch/flow,samwgoldman/flow,popham/flow,jamesgpearce/flow,JonathanUsername/flow,claudiopro/flow,jamesgpearce/flow,jamesgpearce/flow,facebook/flow,popham/flow,gabelevi/flow,samwgoldman/flow,nmote/flow,nmote/flow,nmote/flow,popham/flow,mroch/flow,gabelevi/flow,jamesgpearce/flow,facebook/flow,popham/flow,claudiopro/flow,mroch/flow,nmote/flow,gabelevi/flow,AgentME/flow,JonathanUsername/flow,claudiopro/flow,gabelevi/flow,mroch/flow,JonathanUsername/flow,AgentME/flow,claudiopro/flow,claudiopro/flow,samwgoldman/flow