Fix ResynthesizerResultProvider's handling of parts.

pkg/analyzer/lib/src/generated/source.dart

 // Copyright (c) 2014, 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.src.generated.source;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 466 matching lines @@
   @override
   int get hashCode;
 
   /**
    * Return `true` if this source is in one of the system libraries.
    *
    * @return `true` if this is in a system library
    */
   bool get isInSystemLibrary;
 
+  @override
+  Source get librarySource => null;
+
   /**
    * Return the modification stamp for this source, or a negative value if the
    * source does not exist. A modification stamp is a non-negative integer with
    * the property that if the contents of the source have not been modified
    * since the last time the modification stamp was accessed then the same value
    * will be returned, but if the contents of the source have been modified one
    * or more times (even if the net change is zero) the stamps will be different.
    *
    * Clients should consider using the method
    * [AnalysisContext.getModificationStamp] because contexts can have local
@@ skipping 433 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]);
 
   /**
    * Return an absolute URI that represents the given [source], or `null` if a
    * valid URI cannot be computed.
    *
    * The computation should be based solely on [source.fullName].
    */
   Uri restoreAbsolute(Source source) => null;
 }