diff --git a/src/cmd/doc/dirs.go b/src/cmd/doc/dirs.go
index 5088f880e1..f5fb795dc7 100644
--- a/src/cmd/doc/dirs.go
+++ b/src/cmd/doc/dirs.go
@@ -24,8 +24,11 @@ type Dirs struct {
 
 var dirs Dirs
 
-func dirsInit() {
+// dirsInit starts the scanning of package directories in GOROOT and GOPATH. Any
+// extra paths passed to it are included in the channel.
+func dirsInit(extra ...string) {
 	dirs.paths = make([]string, 0, 1000)
+	dirs.paths = append(dirs.paths, extra...)
 	dirs.scan = make(chan string)
 	go dirs.walk()
 }
@@ -97,8 +100,9 @@ func (d *Dirs) bfsWalkRoot(root string) {
 					continue
 				}
 				// Entry is a directory.
-				// No .git or other dot nonsense please.
-				if strings.HasPrefix(name, ".") {
+
+				// The go tool ignores directories starting with ., _, or named "testdata".
+				if name[0] == '.' || name[0] == '_' || name == "testdata" {
 					continue
 				}
 				// Remember this (fully qualified) directory for the next pass.
diff --git a/src/cmd/doc/doc_test.go b/src/cmd/doc/doc_test.go
index e68fb017b9..f1072b5e41 100644
--- a/src/cmd/doc/doc_test.go
+++ b/src/cmd/doc/doc_test.go
@@ -16,10 +16,18 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	// otherwise the tests are brittle, as they may give unexpected
-	// output or errors when a suffix match with GOPATH takes place
+	// Clear GOPATH so we don't access the user's own packages in the test.
 	buildCtx.GOPATH = ""
-	dirsInit()
+
+	// Add $GOROOT/src/cmd/doc/testdata explicitly so we can access its contents in the test.
+	// Normally testdata directories are ignored, but sending it to dirs.scan directly is
+	// a hack that works around the check.
+	testdataDir, err := filepath.Abs("testdata")
+	if err != nil {
+		panic(err)
+	}
+	dirsInit(testdataDir)
+
 	os.Exit(m.Run())
 }