Reason for government pushing a pre-Brexit election, apart from believing they can win, is to avoid any risk of the deal being voted down or having to make concessions to get it through.
Making concessions to domestic groups to pass international treaties like trade deals is entirely normal, but unfortunately this has not been established in the UK political narrative, where it is assumed wrongly that government majority = treaty approval.
It would actually be a healthy process for Parliament to go through the Withdrawal Agreement properly, voting on amendments, and allowing the full range of arguments. It would probably improve the bill and the chances of success in Brexit phase 2. But it looks unlikely.
Blogged - Brexit extension shows the UK continues to struggle with the basics of international negotiations ecipe.org/blog/brexit-ex… 
