Add golang/x/tools (godoc/vfs) as a dependency

Gopkg.lock

@@ -406,6 +406,15 @@
   revision = "f21a4dfb5e38f5895301dc265a8def02365cc3d0"
   version = "v0.3.0"
 
+[[projects]]
+  branch = "master"
+  name = "golang.org/x/tools"
+  packages = [
+    "godoc/vfs",
+    "godoc/vfs/mapfs"
+  ]
+  revision = "c06a8d8ed11aae27b08c1c109a509ada9de81240"
+
 [[projects]]
   branch = "master"
   name = "google.golang.org/api"
@@ -500,6 +509,6 @@
 [solve-meta]
   analyzer-name = "dep"
   analyzer-version = 1
-  inputs-digest = "48c51d905f8ddc56632c559e57973899a121d6056d00d31189b78b91fb19df13"
+  inputs-digest = "952240f227fe19a7d62faa43d3acad05b097803337542117c1c3bd81cc5ffd6d"
   solver-name = "gps-cdcl"
   solver-version = 1

Gopkg.toml

@@ -85,6 +85,10 @@
   name = "cloud.google.com/go"
   version = "0.23.0"
 
+[[override]]
+  branch = "master"
+  name = "golang.org/x/tools"
+
 [prune]
   go-tests = true
   unused-packages = true