Update the protocol buffer compiler plug-in library to the latest Dart SDK

lib/extension_generator.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of protoc;
 
 class ExtensionGenerator implements ProtobufContainer {
   final String fqname;
   final FieldDescriptorProto _descriptor;
   final ProtobufContainer _parent;
@@ skipping 51 matching lines @@
         builder = ',${SP}()${SP}=>${SP}new ${typeName}()';
       }
     } else {
       if (_descriptor.label == FieldDescriptorProto_Label.LABEL_REPEATED) {
         initializer = ',${SP}()${SP}=>${SP}new PbList<${baseType}>()';
       } else if (field.hasInitialization) {
         initializer = ',${SP}${field.initialization}';
       }
     }
 
-    if (field.enum) {
+    if (field.enm) {
       if (initializer.isEmpty) {
         initializer = ',${SP}null';
       }
       if (builder.isEmpty) {
         builder = ',${SP}null';
       }
       valueOf = ',${SP}(var v)${SP}=>${SP}${field.baseType}.valueOf(v)';
     }
 
     out.println('static final Extension $name${SP}=${SP}'
       'new Extension(\'$extendee\',${SP}\'$name\',${SP}'
       '${_descriptor.number},${SP}GeneratedMessage.$type'
       '${initializer}${builder}${valueOf}'
       ');');
   }
 
   String get name => classname;
 }