use go1.11 with go mod

vendor/github.com/stretchr/testify/assert/errors.go

@@ -1,10 +0,0 @@
-package assert
-
-import (
-	"errors"
-)
-
-// AnError is an error instance useful for testing.  If the code does not care
-// about error specifics, and only needs to return the error for example, this
-// error should be used to make the test code more readable.
-var AnError = errors.New("assert.AnError general error for testing")