internal/impl: change Go representation of extension lists to []T

Change-Id: Iebcefe0330c8f858c7735f9362abfd87043ee39d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/192458
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/encoding/prototext/encode_test.go b/encoding/prototext/encode_test.go
index 0c480e6..15e98a1 100644
--- a/encoding/prototext/encode_test.go
+++ b/encoding/prototext/encode_test.go
@@ -969,9 +969,9 @@
 		desc: "extensions of repeated fields",
 		input: func() proto.Message {
 			m := &pb2.Extensions{}
-			proto.SetExtension(m, pb2.E_RptExtEnum, &[]pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
-			proto.SetExtension(m, pb2.E_RptExtFixed32, &[]uint32{42, 47})
-			proto.SetExtension(m, pb2.E_RptExtNested, &[]*pb2.Nested{
+			proto.SetExtension(m, pb2.E_RptExtEnum, []pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
+			proto.SetExtension(m, pb2.E_RptExtFixed32, []uint32{42, 47})
+			proto.SetExtension(m, pb2.E_RptExtNested, []*pb2.Nested{
 				&pb2.Nested{OptString: proto.String("one")},
 				&pb2.Nested{OptString: proto.String("two")},
 				&pb2.Nested{OptString: proto.String("three")},
@@ -1026,9 +1026,9 @@
 				OptBool:   proto.Bool(true),
 				OptInt32:  proto.Int32(42),
 			}
-			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtEnum, &[]pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
-			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtString, &[]string{"hello", "world"})
-			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtNested, &[]*pb2.Nested{
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtEnum, []pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtString, []string{"hello", "world"})
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtNested, []*pb2.Nested{
 				&pb2.Nested{OptString: proto.String("one")},
 				&pb2.Nested{OptString: proto.String("two")},
 				&pb2.Nested{OptString: proto.String("three")},