missing virtual on MergeFrom

src/ProtocolBuffers.sln

@@ -22,6 +22,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "proto", "proto", "{1F896D5C
 		..\protos\tutorial\addressbook.proto = ..\protos\tutorial\addressbook.proto
 		..\protos\google\protobuf\csharp_options.proto = ..\protos\google\protobuf\csharp_options.proto
 		..\protos\google\protobuf\descriptor.proto = ..\protos\google\protobuf\descriptor.proto
+		..\ProtocolBuffers.build = ..\ProtocolBuffers.build
 		..\todo.txt = ..\todo.txt
 	EndProjectSection
 EndProject

src/ProtocolBuffers/AbstractBuilder.cs

@@ -263,7 +263,7 @@ namespace Google.ProtocolBuffers {
       return MergeFrom((IMessage)other);
     }
 
-    public TBuilder MergeFrom(CodedInputStream input, ExtensionRegistryLite extensionRegistry) {
+    public virtual TBuilder MergeFrom(CodedInputStream input, ExtensionRegistryLite extensionRegistry) {
 #warning Not implemented for Lite edition
       return MergeFrom(input, (ExtensionRegistry)extensionRegistry);
     }