Add VariableDefinition Visitor to graphql-ir
Add VariableDefinition Visitor to graphql-ir

Reviewed By: kassens

Differential Revision: D24651739

fbshipit-source-id: c7b5e3fabe6c8b3d43b3c6d17dde472803ce5b23

voideanvalue/relay,facebook/relay,yungsters/relay,kassens/relay,facebook/relay,facebook/relay,wincent/relay,wincent/relay,wincent/relay,xuorig/relay,voideanvalue/relay,wincent/relay,josephsavona/relay,kassens/relay,xuorig/relay,atxwebs/relay,wincent/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,voideanvalue/relay,yungsters/relay,kassens/relay,xuorig/relay,atxwebs/relay,kassens/relay,voideanvalue/relay,josephsavona/relay,facebook/relay,atxwebs/relay,kassens/relay,atxwebs/relay,wincent/relay,facebook/relay,josephsavona/relay,kassens/relay,xuorig/relay,yungsters/relay,xuorig/relay,facebook/relay