Add entrypoint to Env_builder like Ssa_builder
Add entrypoint to Env_builder like Ssa_builder

Summary: We'll need a way of testing the output of the EnvBuilder. This diff copies the same strategy that the SSA builder uses for an entrypoint into the analysis

Reviewed By: panagosg7

Differential Revision: D27608541

fbshipit-source-id: 34952f54849292e32d45b58ae7e6f9c6b93f5d43

mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow