Fix ResynthesizerResultProvider's handling of parts.

pkg/analyzer/test/src/context/cache_test.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.test.src.context.cache_test;
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/sdk_io.dart';
@@ skipping 1170 matching lines @@
       ResultDescriptor descriptor, Object value) {
     if (descriptor == keepDescriptor) {
       return DeltaResult.KEEP_CONTINUE;
     }
     return DeltaResult.INVALIDATE;
   }
 }
 
 class _TestAnalysisTarget implements AnalysisTarget {
   @override
+  Source get librarySource => null;
+
+  @override
   Source get source => null;
 }