cmd/api: use one token.FileSet for all contexts
cmd/api: use one token.FileSet for all contexts

Prep for future CL.

R=gri
CC=golang-dev
http://codereview.appspot.com/6843048

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang