all: remove dependency on proto v1

This does not remove all dependencies,
but all of the cases where it can now be implemented in terms of v2.

Change-Id: Idc5b0273f0d35c284bf2141eb9cce998692ceb15
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/184878
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/encoding/prototext/other_test.go b/encoding/prototext/other_test.go
index ec1842f..5b3376b 100644
--- a/encoding/prototext/other_test.go
+++ b/encoding/prototext/other_test.go
@@ -7,7 +7,6 @@
 import (
 	"testing"
 
-	protoV1 "github.com/golang/protobuf/proto"
 	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/internal/impl"
 	pimpl "google.golang.org/protobuf/internal/impl"
@@ -240,7 +239,7 @@
 				t.Errorf("Unmarshal() returned error: %v\n\n", err)
 			}
 
-			if !protoV1.Equal(gotMessage, tt.message.(protoV1.Message)) {
+			if !proto.Equal(gotMessage, tt.message) {
 				t.Errorf("Unmarshal()\n<got>\n%v\n<want>\n%v\n", gotMessage, tt.message)
 			}
 		})