Dockerfile: use latest x/tools containing the CL 113875

Use the godoc with the CL 113875. That CL handles the case when there
was no output from the user program and vet returned an error.

Fixes golang/go#27980

Change-Id: I8f3d5ff477fb930c099fd9780c102c8421403e55
Reviewed-on: https://go-review.googlesource.com/c/140017
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/Dockerfile b/Dockerfile
index 41a0849..6e3d138 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -72,8 +72,8 @@
 RUN go get -d golang.org/x/text/secure/bidirule `#and 4 other pkgs` &&\
     (cd /go/src/golang.org/x/text && (git cat-file -t $REV 2>/dev/null || git fetch -q origin $REV) && git reset --hard $REV)
 
-# Repo golang.org/x/tools at 48418e5 (2018-05-08)
-ENV REV=48418e5732e1b1e2a10207c8007a5f959e422f20
+# Repo golang.org/x/tools at 59602fd (2018-10-04)
+ENV REV=59602fdee893255faef9b2cd3aa8f880ea85265c
 RUN go get -d golang.org/x/tools/go/ast/astutil `#and 3 other pkgs` &&\
     (cd /go/src/golang.org/x/tools && (git cat-file -t $REV 2>/dev/null || git fetch -q origin $REV) && git reset --hard $REV)