exp/eval: fix compile error.
exp/eval: fix compile error.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4550086

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,ssrl/go,gnanderson/go,dsriram/golang,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,ssrl/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,dsriram/golang,ssrl/go,apparentlymart/golang,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,dsriram/golang,ssrl/go,apparentlymart/golang,dsriram/golang