Replace Map get/set with update
Replace Map get/set with update

Summary: It looks like `update` didn't exist when this was written, but in my opinion it's more elegant and it also saves a map traversal.

Reviewed By: jbrown215

Differential Revision: D26056879

fbshipit-source-id: a7d3beb4cc71d6da13c6154ebfc5c899e5f8bb7a

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow