Update protos from protocolbuffers/protobuf.

Using f425b9f059a57dc5f13a6e6a389fff1a6cc06749 as reference point.

Author: thomasvl

Protos/google/protobuf/compiler/plugin.proto

@@ -45,6 +45,7 @@
 // flag "--${NAME}_out" is passed to protoc.
 
 syntax = "proto2";
+
 package google.protobuf.compiler;
 option java_package = "com.google.protobuf.compiler";
 option java_outer_classname = "PluginProtos";

Protos/google/protobuf/test_messages_proto2.proto

@@ -33,6 +33,8 @@
 // - conformance tests
 //
 
+// LINT: ALLOW_GROUPS
+
 syntax = "proto2";
 
 package protobuf_test_messages.proto2;
@@ -180,6 +182,9 @@ message TestAllTypesProto2 {
   optional int32 field_name17__ = 417;
   optional int32 Field_name18__ = 418;
 
+  // Reserved for unknown fields test.
+  reserved 1000 to 9999;
+
   // message_set test case.
   message MessageSetCorrect {
     option message_set_wire_format = true;
@@ -214,3 +219,15 @@ enum ForeignEnumProto2 {
 extend TestAllTypesProto2 {
   optional int32 extension_int32 = 120;
 }
+
+message UnknownToTestAllTypes {
+  optional int32  optional_int32 = 1001;
+  optional string optional_string = 1002;
+  optional ForeignMessageProto2 nested_message = 1003;
+  optional group OptionalGroup = 1004 {
+    optional int32 a = 1;
+  }
+  optional bool optional_bool = 1006;
+  repeated int32 repeated_int32 = 1011;
+}
+