Use package names as import prefixes when generating code

test/message_generator_test.dart

 #!/usr/bin/env 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.
 
 library message_generator_test;
 
 import 'package:protoc-plugin/src/descriptor.pb.dart';
 import 'package:protoc-plugin/src/plugin.pb.dart';
 import 'package:protoc-plugin/protoc.dart';
@@ skipping 45 matching lines @@
   bool hasType() => hasField(2);
   void clearType() => clearField(2);
 
   String get name => getField(3);
   void set name(String v) { setField(3, v); }
   bool hasName() => hasField(3);
   void clearName() => clearField(3);
 }
 
 ''';
+    FileDescriptorProto fd = new FileDescriptorProto();
     EnumDescriptorProto ed = new EnumDescriptorProto()
         ..name = 'PhoneType'
         ..value.addAll([
             new EnumValueDescriptorProto()
                 ..name = 'MOBILE'
                 ..number = 0,
             new EnumValueDescriptorProto()
                 ..name = 'HOME'
                 ..number = 1,
             new EnumValueDescriptorProto()
@@ skipping 10 matching lines @@
                 ..name = 'number'
                 ..number = 1
                 ..label = FieldDescriptorProto_Label.LABEL_REQUIRED
                 ..type = FieldDescriptorProto_Type.TYPE_STRING,
             // optional PhoneType type = 2 [default = HOME];
             new FieldDescriptorProto()
                 ..name = 'type'
                 ..number = 2
                 ..label = FieldDescriptorProto_Label.LABEL_OPTIONAL
                 ..type = FieldDescriptorProto_Type.TYPE_ENUM
-                ..typeName = 'PhoneNumber.PhoneType',
+                ..typeName = '.PhoneNumber.PhoneType',
             new FieldDescriptorProto()
                 ..name = 'name'
                 ..number = 3
                 ..label = FieldDescriptorProto_Label.LABEL_OPTIONAL
                 ..type = FieldDescriptorProto_Type.TYPE_STRING
                 ..defaultValue = r'$'
             ])
         ..enumType.add(ed);
     MemoryWriter buffer = new MemoryWriter();
     IndentingWriter writer = new IndentingWriter('  ', buffer);
     var options =
         new GenerationOptions(
             new CodeGeneratorRequest(), new CodeGeneratorResponse());
-    MessageGenerator mg =
-        new MessageGenerator(
-            md, null, new GenerationContext(options));
+    var context = new GenerationContext(options);
+    FileGenerator fg = new FileGenerator(fd, null, context);
+    MessageGenerator mg = new MessageGenerator(md, fg, context);
     mg.initializeFields();
     mg.generate(writer);
     expect(buffer.toString(), expected);
   });
 }