Restore the navigation test for an unresolved part URI.

pkg/analysis_server/test/analysis_notification_navigation_test.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 test.domain.analysis.notification.navigation;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/computer/element.dart';
@@ skipping 428 matching lines @@
 library lib;
 part "test_unit.dart";
 ''');
     return prepareNavigation(() {
       assertHasRegionString('part "test_unit.dart"');
       assertHasFileTarget(unitFile, 0, 0);
     });
   }
 
   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() {
     addTestFile('''
 class AAA {}
 main() {
   AAA aaa = null;
 }
 ''');
     return prepareNavigation(() {
@@ skipping 13 matching lines @@
   final int length;
   final List<Element> targets;
 
   NavigationRegion(this.offset, this.length, this.targets);
 
   @override
   String toString() {
     return 'NavigationRegion(offset=$offset; length=$length; targets=$targets';
   }
 }