Rename mixin protos and add a check for undefined mixin name

lib/linker.dart

Index: lib/linker.dart
diff --git a/lib/linker.dart b/lib/linker.dart
index 5bca6d92177b34f4208b91b3a234153e97afac91..fa1ab9de9e75f4ad3349c7b852f27bd3eeb36bb0 100644
--- a/lib/linker.dart
+++ b/lib/linker.dart
@@ -48,7 +48,7 @@ class GenerationContext {
   /// Makes info about a .pb.dart file available for reference,
   /// using the filename given to us by protoc.
   void registerProtoFile(FileGenerator f) {
-    _files[f._fileDescriptor.name] = f;
+    _files[f.descriptor.name] = f;
   }
 
   /// Makes a message, group, or enum available for reference.