encoding/textpb: switch tests to use V2 generated messages

Change-Id: I817568aec5fbf053c3566b311e92b79aff1caf7e
Reviewed-on: https://go-review.googlesource.com/c/154177
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/textpb/encode_test.go b/encoding/textpb/encode_test.go
index 7c9a912..55f3e77 100644
--- a/encoding/textpb/encode_test.go
+++ b/encoding/textpb/encode_test.go
@@ -9,11 +9,9 @@
 	"strings"
 	"testing"
 
-	protoV1 "github.com/golang/protobuf/proto"
 	"github.com/golang/protobuf/v2/encoding/textpb"
 	"github.com/golang/protobuf/v2/internal/detrand"
 	"github.com/golang/protobuf/v2/internal/encoding/pack"
-	"github.com/golang/protobuf/v2/internal/impl"
 	"github.com/golang/protobuf/v2/internal/scalar"
 	"github.com/golang/protobuf/v2/proto"
 	"github.com/google/go-cmp/cmp"
@@ -32,10 +30,6 @@
 	detrand.Disable()
 }
 
-func M(m interface{}) proto.Message {
-	return impl.Export{}.MessageOf(m).Interface()
-}
-
 // splitLines is a cmpopts.Option for comparing strings with line breaks.
 var splitLines = cmpopts.AcyclicTransformer("SplitLines", func(s string) []string {
 	return strings.Split(s, "\n")
@@ -56,7 +50,7 @@
 func TestMarshal(t *testing.T) {
 	tests := []struct {
 		desc    string
-		input   protoV1.Message
+		input   proto.Message
 		want    string
 		wantErr bool
 	}{{
@@ -806,7 +800,7 @@
 		tt := tt
 		t.Run(tt.desc, func(t *testing.T) {
 			t.Parallel()
-			b, err := textpb.Marshal(M(tt.input))
+			b, err := textpb.Marshal(tt.input)
 			if err != nil && !tt.wantErr {
 				t.Errorf("Marshal() returned error: %v\n\n", err)
 			}