Handle protocol buffer messages with no package

lib/file_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 FileGenerator extends ProtobufContainer {
   final FileDescriptorProto _fileDescriptor;
   final ProtobufContainer _parent;
   final GenerationContext _context;
@@ skipping 115 matching lines @@
       if (importPath.isAbsolute) {
         // protoc should never generate an import with an absolute path.
         throw("FAILURE: Import with absolute path is not supported");
       }
       // Create a relative path from the current file to the import.
       Uri relativeProtoPath = _relative(importPath, filePath);
       // Find the file generator for this import as it contains the
       // package name.
       FileGenerator fileGenerator = _context.lookupFile(import);
       out.print("import '${_generatedFilePath(relativeProtoPath)}'");
-      if (package != fileGenerator.package) {
+      if (package != fileGenerator.package && !fileGenerator.package.isEmpty) {
         out.print(' as ${fileGenerator.packageImportPrefix}');
       }
       out.println(';');
     }
     out.println('');
 
     // Initialize Field.
     for (MessageGenerator m in messageGenerators) {
       m.initializeFields();
     }
@@ skipping 39 matching lines @@
     _registry[container.fqname] = container;
     if (container is FileGenerator) {
       _files[container._fileDescriptor.name] = container;
     }
   }
 
   ProtobufContainer operator [](String fqname) => _registry[fqname];
 
   FileGenerator lookupFile(String name) => _files[name];
 }