exp/types: avoid init race in check_test.go.
exp/types: avoid init race in check_test.go.

There was an init race between
	check_test.go:init
        universe.go:def
        use of Universe
and
	universe.go:init
        creation of Universe

The order in which init funcs are executed in a package is unspecified.
The test is not currently broken in the golang.org environment
because the go tool compiles the test with non-test sources before test sources,
but other environments may, say, sort the source files before compiling,
and thus trigger this race, causing a nil pointer panic.

R=gri
CC=golang-dev
http://codereview.appspot.com/6827076

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