First part of implementing wrapper types. Not ready yet!

BUILD

diff --git a/BUILD b/BUILD
index 5177e43..5cfed33 100644
--- a/BUILD
+++ b/BUILD
@@ -162,10 +162,10 @@
         "src/google/protobuf/compiler/cpp/cpp_string_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_enum.cc",
         "src/google/protobuf/compiler/csharp/csharp_enum_field.cc",
-        "src/google/protobuf/compiler/csharp/csharp_extension.cc",
         "src/google/protobuf/compiler/csharp/csharp_field_base.cc",
         "src/google/protobuf/compiler/csharp/csharp_generator.cc",
         "src/google/protobuf/compiler/csharp/csharp_helpers.cc",
+        "src/google/protobuf/compiler/csharp/csharp_map_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_message.cc",
         "src/google/protobuf/compiler/csharp/csharp_message_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_primitive_field.cc",
@@ -174,7 +174,7 @@
         "src/google/protobuf/compiler/csharp/csharp_repeated_primitive_field.cc",
         "src/google/protobuf/compiler/csharp/csharp_source_generator_base.cc",
         "src/google/protobuf/compiler/csharp/csharp_umbrella_class.cc",
-        "src/google/protobuf/compiler/csharp/csharp_writer.cc",
+        "src/google/protobuf/compiler/csharp/csharp_wrapper_field.cc",
         "src/google/protobuf/compiler/java/java_context.cc",
         "src/google/protobuf/compiler/java/java_doc_comment.cc",
         "src/google/protobuf/compiler/java/java_enum.cc",