Fix issue with importing packages where the package name included a dot

lib/enum_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 EnumAlias {
   final EnumValueDescriptorProto value;
   final EnumValueDescriptorProto canonicalValue;
   EnumAlias(this.value, this.canonicalValue);
 }
 
-class EnumGenerator implements ProtobufContainer {
+class EnumGenerator extends ProtobufContainer {
   final ProtobufContainer _parent;
   final String classname;
   final String fqname;
   final List<EnumValueDescriptorProto> _canonicalValues =
       <EnumValueDescriptorProto>[];
   final List<EnumAlias> _aliases = <EnumAlias>[];
 
   EnumGenerator(
       EnumDescriptorProto descriptor,
       ProtobufContainer parent,
@@ skipping 49 matching lines @@
           '${SP}ProtobufEnum.initByValue(values);');
       out.println('static ${classname} valueOf(int value)${SP}=>'
           '${SP}_byValue[value];');
       out.println();
 
       out.println('const ${classname}._(int v,${SP}String n)${SP}'
           ':${SP}super(v,${SP}n);');
     });
   }
 }