Rename mixin protos and add a check for undefined mixin name

lib/code_generator.dart

Index: lib/code_generator.dart
diff --git a/lib/code_generator.dart b/lib/code_generator.dart
index 2f3f92cc664804641c815f533e1c35013cfbf53a..9c9182ee61dc92e5ba372abb52bc0faeb6ecf083 100644
--- a/lib/code_generator.dart
+++ b/lib/code_generator.dart
@@ -63,7 +63,7 @@ class CodeGenerator extends ProtobufContainer {
 
       // Generate the .pb.dart file if requested.
       for (var gen in generators) {
-        var name = gen._fileDescriptor.name;
+        var name = gen.descriptor.name;
         if (request.fileToGenerate.contains(name)) {
           response.file.addAll(gen.generateFiles(config));
         }