Fix ResynthesizerResultProvider's handling of parts.

pkg/analyzer/lib/task/model.dart

Index: pkg/analyzer/lib/task/model.dart
diff --git a/pkg/analyzer/lib/task/model.dart b/pkg/analyzer/lib/task/model.dart
index 4f64bc9a6dc404739a53ac673a8ca6bfe6f596a1..771d4227d89222815aa94dc2a565e13c99071279 100644
--- a/pkg/analyzer/lib/task/model.dart
+++ b/pkg/analyzer/lib/task/model.dart
@@ -61,6 +61,9 @@ class AnalysisContextTarget implements AnalysisTarget {
   AnalysisContextTarget(this.context);
 
   @override
+  Source get librarySource => null;
+
+  @override
   Source get source => null;
 }
 
@@ -73,6 +76,12 @@ class AnalysisContextTarget implements AnalysisTarget {
  */
 abstract class AnalysisTarget {
   /**
+   * If this target is associated with a library, return the source of the
+   * library's defining compilation unit; otherwise return `null`.
+   */
+  Source get librarySource;
+
+  /**
    * Return the source associated with this target, or `null` if this target is
    * not associated with a source.
    */