diff --git a/protoc-gen-go/testdata/Makefile b/protoc-gen-go/testdata/Makefile
index a85cc56..105cdec 100644
--- a/protoc-gen-go/testdata/Makefile
+++ b/protoc-gen-go/testdata/Makefile
@@ -47,6 +47,7 @@
 	make -B my_test/test.pb.go
 	sed -i '/return.*fileDescriptor/d' my_test/test.pb.go
 	sed -i '/^var fileDescriptor/,/^}/d' my_test/test.pb.go
+	sed -i '/proto.RegisterFile.*fileDescriptor/d' my_test/test.pb.go
 	gofmt -w my_test/test.pb.go
 	diff -w my_test/test.pb.go my_test/test.pb.go.golden
 
diff --git a/protoc-gen-go/testdata/my_test/test.pb.go b/protoc-gen-go/testdata/my_test/test.pb.go
index 997743b..9ec3e12 100644
--- a/protoc-gen-go/testdata/my_test/test.pb.go
+++ b/protoc-gen-go/testdata/my_test/test.pb.go
@@ -33,7 +33,9 @@
 
 // This is a compile-time assertion to ensure that this generated file
 // is compatible with the proto package it is being compiled against.
-const _ = proto.ProtoPackageIsVersion1
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
 
 type HatType int32
 
@@ -282,10 +284,10 @@
 }
 
 type Reply struct {
-	Found            []*Reply_Entry            `protobuf:"bytes,1,rep,name=found" json:"found,omitempty"`
-	CompactKeys      []int32                   `protobuf:"varint,2,rep,packed,name=compact_keys,json=compactKeys" json:"compact_keys,omitempty"`
-	XXX_extensions   map[int32]proto.Extension `json:"-"`
-	XXX_unrecognized []byte                    `json:"-"`
+	Found                        []*Reply_Entry `protobuf:"bytes,1,rep,name=found" json:"found,omitempty"`
+	CompactKeys                  []int32        `protobuf:"varint,2,rep,packed,name=compact_keys,json=compactKeys" json:"compact_keys,omitempty"`
+	proto.XXX_InternalExtensions `json:"-"`
+	XXX_unrecognized             []byte `json:"-"`
 }
 
 func (m *Reply) Reset()         { *m = Reply{} }
@@ -299,12 +301,6 @@
 func (*Reply) ExtensionRangeArray() []proto.ExtensionRange {
 	return extRange_Reply
 }
-func (m *Reply) ExtensionMap() map[int32]proto.Extension {
-	if m.XXX_extensions == nil {
-		m.XXX_extensions = make(map[int32]proto.Extension)
-	}
-	return m.XXX_extensions
-}
 
 func (m *Reply) GetFound() []*Reply_Entry {
 	if m != nil {
@@ -355,9 +351,9 @@
 }
 
 type OtherBase struct {
-	Name             *string                   `protobuf:"bytes,1,opt,name=name" json:"name,omitempty"`
-	XXX_extensions   map[int32]proto.Extension `json:"-"`
-	XXX_unrecognized []byte                    `json:"-"`
+	Name                         *string `protobuf:"bytes,1,opt,name=name" json:"name,omitempty"`
+	proto.XXX_InternalExtensions `json:"-"`
+	XXX_unrecognized             []byte `json:"-"`
 }
 
 func (m *OtherBase) Reset()         { *m = OtherBase{} }
@@ -371,12 +367,6 @@
 func (*OtherBase) ExtensionRangeArray() []proto.ExtensionRange {
 	return extRange_OtherBase
 }
-func (m *OtherBase) ExtensionMap() map[int32]proto.Extension {
-	if m.XXX_extensions == nil {
-		m.XXX_extensions = make(map[int32]proto.Extension)
-	}
-	return m.XXX_extensions
-}
 
 func (m *OtherBase) GetName() string {
 	if m != nil && m.Name != nil {
@@ -434,8 +424,8 @@
 }
 
 type OldReply struct {
-	XXX_extensions   map[int32]proto.Extension `json:"-"`
-	XXX_unrecognized []byte                    `json:"-"`
+	proto.XXX_InternalExtensions `json:"-"`
+	XXX_unrecognized             []byte `json:"-"`
 }
 
 func (m *OldReply) Reset()         { *m = OldReply{} }
@@ -443,16 +433,16 @@
 func (*OldReply) ProtoMessage()    {}
 
 func (m *OldReply) Marshal() ([]byte, error) {
-	return proto.MarshalMessageSet(m.ExtensionMap())
+	return proto.MarshalMessageSet(&m.XXX_InternalExtensions)
 }
 func (m *OldReply) Unmarshal(buf []byte) error {
-	return proto.UnmarshalMessageSet(buf, m.ExtensionMap())
+	return proto.UnmarshalMessageSet(buf, &m.XXX_InternalExtensions)
 }
 func (m *OldReply) MarshalJSON() ([]byte, error) {
-	return proto.MarshalMessageSetJSON(m.XXX_extensions)
+	return proto.MarshalMessageSetJSON(&m.XXX_InternalExtensions)
 }
 func (m *OldReply) UnmarshalJSON(buf []byte) error {
-	return proto.UnmarshalMessageSetJSON(buf, m.XXX_extensions)
+	return proto.UnmarshalMessageSetJSON(buf, &m.XXX_InternalExtensions)
 }
 
 // ensure OldReply satisfies proto.Marshaler and proto.Unmarshaler
@@ -466,12 +456,6 @@
 func (*OldReply) ExtensionRangeArray() []proto.ExtensionRange {
 	return extRange_OldReply
 }
-func (m *OldReply) ExtensionMap() map[int32]proto.Extension {
-	if m.XXX_extensions == nil {
-		m.XXX_extensions = make(map[int32]proto.Extension)
-	}
-	return m.XXX_extensions
-}
 
 type Communique struct {
 	MakeMeCry *bool `protobuf:"varint,1,opt,name=make_me_cry,json=makeMeCry" json:"make_me_cry,omitempty"`
diff --git a/protoc-gen-go/testdata/my_test/test.pb.go.golden b/protoc-gen-go/testdata/my_test/test.pb.go.golden
index 997743b..9ec3e12 100644
--- a/protoc-gen-go/testdata/my_test/test.pb.go.golden
+++ b/protoc-gen-go/testdata/my_test/test.pb.go.golden
@@ -33,7 +33,9 @@
 
 // This is a compile-time assertion to ensure that this generated file
 // is compatible with the proto package it is being compiled against.
-const _ = proto.ProtoPackageIsVersion1
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
 
 type HatType int32
 
@@ -282,10 +284,10 @@
 }
 
 type Reply struct {
-	Found            []*Reply_Entry            `protobuf:"bytes,1,rep,name=found" json:"found,omitempty"`
-	CompactKeys      []int32                   `protobuf:"varint,2,rep,packed,name=compact_keys,json=compactKeys" json:"compact_keys,omitempty"`
-	XXX_extensions   map[int32]proto.Extension `json:"-"`
-	XXX_unrecognized []byte                    `json:"-"`
+	Found                        []*Reply_Entry `protobuf:"bytes,1,rep,name=found" json:"found,omitempty"`
+	CompactKeys                  []int32        `protobuf:"varint,2,rep,packed,name=compact_keys,json=compactKeys" json:"compact_keys,omitempty"`
+	proto.XXX_InternalExtensions `json:"-"`
+	XXX_unrecognized             []byte `json:"-"`
 }
 
 func (m *Reply) Reset()         { *m = Reply{} }
@@ -299,12 +301,6 @@
 func (*Reply) ExtensionRangeArray() []proto.ExtensionRange {
 	return extRange_Reply
 }
-func (m *Reply) ExtensionMap() map[int32]proto.Extension {
-	if m.XXX_extensions == nil {
-		m.XXX_extensions = make(map[int32]proto.Extension)
-	}
-	return m.XXX_extensions
-}
 
 func (m *Reply) GetFound() []*Reply_Entry {
 	if m != nil {
@@ -355,9 +351,9 @@
 }
 
 type OtherBase struct {
-	Name             *string                   `protobuf:"bytes,1,opt,name=name" json:"name,omitempty"`
-	XXX_extensions   map[int32]proto.Extension `json:"-"`
-	XXX_unrecognized []byte                    `json:"-"`
+	Name                         *string `protobuf:"bytes,1,opt,name=name" json:"name,omitempty"`
+	proto.XXX_InternalExtensions `json:"-"`
+	XXX_unrecognized             []byte `json:"-"`
 }
 
 func (m *OtherBase) Reset()         { *m = OtherBase{} }
@@ -371,12 +367,6 @@
 func (*OtherBase) ExtensionRangeArray() []proto.ExtensionRange {
 	return extRange_OtherBase
 }
-func (m *OtherBase) ExtensionMap() map[int32]proto.Extension {
-	if m.XXX_extensions == nil {
-		m.XXX_extensions = make(map[int32]proto.Extension)
-	}
-	return m.XXX_extensions
-}
 
 func (m *OtherBase) GetName() string {
 	if m != nil && m.Name != nil {
@@ -434,8 +424,8 @@
 }
 
 type OldReply struct {
-	XXX_extensions   map[int32]proto.Extension `json:"-"`
-	XXX_unrecognized []byte                    `json:"-"`
+	proto.XXX_InternalExtensions `json:"-"`
+	XXX_unrecognized             []byte `json:"-"`
 }
 
 func (m *OldReply) Reset()         { *m = OldReply{} }
@@ -443,16 +433,16 @@
 func (*OldReply) ProtoMessage()    {}
 
 func (m *OldReply) Marshal() ([]byte, error) {
-	return proto.MarshalMessageSet(m.ExtensionMap())
+	return proto.MarshalMessageSet(&m.XXX_InternalExtensions)
 }
 func (m *OldReply) Unmarshal(buf []byte) error {
-	return proto.UnmarshalMessageSet(buf, m.ExtensionMap())
+	return proto.UnmarshalMessageSet(buf, &m.XXX_InternalExtensions)
 }
 func (m *OldReply) MarshalJSON() ([]byte, error) {
-	return proto.MarshalMessageSetJSON(m.XXX_extensions)
+	return proto.MarshalMessageSetJSON(&m.XXX_InternalExtensions)
 }
 func (m *OldReply) UnmarshalJSON(buf []byte) error {
-	return proto.UnmarshalMessageSetJSON(buf, m.XXX_extensions)
+	return proto.UnmarshalMessageSetJSON(buf, &m.XXX_InternalExtensions)
 }
 
 // ensure OldReply satisfies proto.Marshaler and proto.Unmarshaler
@@ -466,12 +456,6 @@
 func (*OldReply) ExtensionRangeArray() []proto.ExtensionRange {
 	return extRange_OldReply
 }
-func (m *OldReply) ExtensionMap() map[int32]proto.Extension {
-	if m.XXX_extensions == nil {
-		m.XXX_extensions = make(map[int32]proto.Extension)
-	}
-	return m.XXX_extensions
-}
 
 type Communique struct {
 	MakeMeCry *bool `protobuf:"varint,1,opt,name=make_me_cry,json=makeMeCry" json:"make_me_cry,omitempty"`