Fix ResynthesizerResultProvider's handling of parts.

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 790fab65fb96714bcd30b54591f33906acc031f5..8f6050f565b69d9b9d9f00534f02f678dd8795d2 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -2867,6 +2867,9 @@ abstract class ElementImpl implements Element {
       getAncestor((element) => element is LibraryElement);
 
   @override
+  Source get librarySource => library?.source;
+
+  @override
   ElementLocation get location {
     if (_cachedLocation == null) {
       if (library == null) {
@@ -6533,6 +6536,9 @@ class MultiplyDefinedElementImpl implements MultiplyDefinedElement {
   Source get source => null;
 
   @override
+  Source get librarySource => null;
+
+  @override
   DartType get type => DynamicTypeImpl.instance;
 
   @override