Restore the navigation test for an unresolved part URI.

pkg/analysis_server/test/analysis_notification_navigation_test.dart

Index: pkg/analysis_server/test/analysis_notification_navigation_test.dart
diff --git a/pkg/analysis_server/test/analysis_notification_navigation_test.dart b/pkg/analysis_server/test/analysis_notification_navigation_test.dart
index a74fc842962c29d460e3a5e9a72dbfd500c8bd4e..f51041c319dd4b4801ce2c1fadeba969d4d46605 100644
--- a/pkg/analysis_server/test/analysis_notification_navigation_test.dart
+++ b/pkg/analysis_server/test/analysis_notification_navigation_test.dart
@@ -446,15 +446,13 @@ part "test_unit.dart";
   }
 
   test_string_part_unresolvedUri() {
-    // TODO(scheglov) why do we throw MemoryResourceException here?
-    // This Source/File does not exist.
-//    addTestFile('''
-//library lib;
-//part "test_unit.dart";
-//''');
-//    return prepareNavigation(() {
-//      assertNoRegionString('part "test_unit.dart"');
-//    });
+    addTestFile('''
+library lib;
+part "test_unit.dart";
+''');
+    return prepareNavigation(() {
+      assertNoRegionString('part "test_unit.dart"');
+    });
   }
 
   test_targetElement() {