Use the package_resolver package.

lib/src/runner/vm/platform.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:path/path.dart' as p;
 import 'package:stream_channel/stream_channel.dart';
 
@@ skipping 51 matching lines @@
 
         import "${p.toUri(p.absolute(path))}" as test;
 
         void main(_, SendPort message) {
           var channel = serializeSuite(() {
             catchIsolateErrors();
             return test.main;
           });
           new IsolateChannel.connectSend(message).pipe(channel);
         }
-      ''', message, packageRoot: p.toUri(_config.packageRoot), checked: true);
+      ''', message, checked: true);
     }
 
     var url = _config.pubServeUrl.resolveUri(
         p.toUri(p.relative(path, from: 'test') + '.vm_test.dart'));
 
     try {
       return await Isolate.spawnUri(url, [], message, checked: true);
     } on IsolateSpawnException catch (error) {
       if (error.message.contains("OS Error: Connection refused") ||
           error.message.contains("The remote computer refused")) {
         throw new LoadException(path,
             "Error getting $url: Connection refused\n"
             'Make sure "pub serve" is running.');
       } else if (error.message.contains("404 Not Found")) {
         throw new LoadException(path,
             "Error getting $url: 404 Not Found\n"
             'Make sure "pub serve" is serving the test/ directory.');
       }
 
       throw new LoadException(path, error);
     }
   }
 }