Generate a XXX_WellKnownType method for NullValue.

protoc-gen-go/generator/generator.go

diff --git a/protoc-gen-go/generator/generator.go b/protoc-gen-go/generator/generator.go
index 48f029d..6f680f5 100644
--- a/protoc-gen-go/generator/generator.go
+++ b/protoc-gen-go/generator/generator.go
@@ -1426,6 +1426,9 @@
 	}
 	indexes = append(indexes, strconv.Itoa(enum.index))
 	g.P("func (", ccTypeName, ") EnumDescriptor() ([]byte, []int) { return fileDescriptor", g.file.index, ", []int{", strings.Join(indexes, ", "), "} }")
+	if enum.file.GetPackage() == "google.protobuf" && enum.GetName() == "NullValue" {
+		g.P("func (", ccTypeName, `) XXX_WellKnownType() string { return "`, enum.GetName(), `" }`)
+	}
 
 	g.P()
 }

ptypes/struct/struct.pb.go

diff --git a/ptypes/struct/struct.pb.go b/ptypes/struct/struct.pb.go
index 004409e..1de4941 100644
--- a/ptypes/struct/struct.pb.go
+++ b/ptypes/struct/struct.pb.go
@@ -50,6 +50,7 @@
 	return proto.EnumName(NullValue_name, int32(x))
 }
 func (NullValue) EnumDescriptor() ([]byte, []int) { return fileDescriptor0, []int{0} }
+func (NullValue) XXX_WellKnownType() string       { return "NullValue" }
 
 // `Struct` represents a structured data value, consisting of fields
 // which map to dynamically typed values. In some languages, `Struct`