all: run go mod tidy

This adds golang.org/x/sync.
Needed by golang.org/x/tools/internal/lsp/source.

Change-Id: Id42e597be74ed6cb1b7c30c172c5efb3f1c0f2b1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/173481
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
diff --git a/go.mod b/go.mod
index 4f4b20e..850cab5 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,6 @@
 module golang.org/x/tools
 
-require golang.org/x/net v0.0.0-20190311183353-d8887717615a
+require (
+	golang.org/x/net v0.0.0-20190311183353-d8887717615a
+	golang.org/x/sync v0.0.0-20190423024810-112230192c58
+)