exp/gotype: fix build breakage due to https://code.google.com/p/go/source/detail?r=ca5e5de48173

Add 'math/big' to blacklist of packages that use shift
operations as yet unsupported by go/types.

(The failure was masked due to local bugfixes in my client.)

R=rsc, bradfitz, bradfitz
CC=golang-dev
https://golang.org/cl/7220057

src/pkg/exp/gotype/gotype_test.go

@@ -150,7 +150,7 @@ var tests = []string{
 	"log/syslog",
 
 	// "math",
-	"math/big",
+	//"math/big",
 	"math/cmplx",
 	"math/rand",