Fix ResynthesizerResultProvider's handling of parts.

pkg/analyzer/lib/task/dart.dart

 // Copyright (c) 2015, 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 analyzer.task.dart;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 149 matching lines @@
    * Initialize a newly created target for the [unit] in the [library].
    */
   LibrarySpecificUnit(this.library, this.unit);
 
   @override
   int get hashCode {
     return JenkinsSmiHash.combine(library.hashCode, unit.hashCode);
   }
 
   @override
+  Source get librarySource => library;
+
+  @override
   Source get source => unit;
 
   @override
   bool operator ==(other) {
     return other is LibrarySpecificUnit &&
         other.library == library &&
         other.unit == unit;
   }
 
   @override
   String toString() => '$unit in $library';
 }