Show the dependender(s) for missing packages.

sdk/lib/_internal/pub/test/get/path/nonexistent_dir_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
 import 'package:path/path.dart' as path;
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('path dependency to non-existent directory', () {
     var badPath = path.join(sandboxDir, "bad_path");
 
     d.dir(appPath, [
       d.appPubspec({
         "foo": {"path": badPath}
       })
     ]).create();
 
-    pubGet(error: "Could not find package 'foo' at '$badPath'.");
+    pubGet(error: """Could not find package foo at "$badPath".
+Depended on by:
+- myapp""");
   });
 }