go/token: add TestRemovedFileFileReturnsNil test

While debugging test issue in the previous CL i noted that we don't have
a proper test for RemoveFile.

Change-Id: I6a6a6964426ed3cf6725a58ec377686c2900c626
Reviewed-on: https://go-review.googlesource.com/c/go/+/705757
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/go/token/position_test.go

@@ -621,3 +621,25 @@ func TestRemoveFileRace(t *testing.T) {
 		start <- struct{}{}
 	}
 }
+
+func TestRemovedFileFileReturnsNil(t *testing.T) {
+	fset := NewFileSet()
+
+	// Create bunch of files.
+	var files []*File
+	for i := range 1000 {
+		f := fset.AddFile("f", -1, (i+1)*100)
+		files = append(files, f)
+	}
+
+	rand.Shuffle(len(files), func(i, j int) {
+		files[i], files[j] = files[j], files[i]
+	})
+
+	for _, f := range files {
+		fset.RemoveFile(f)
+		if got := fset.File(Pos(f.Base()) + 10); got != nil {
+			t.Fatalf("file was not removed correctly; got file with base: %v", got.Base())
+		}
+	}
+}