Fix test.py and tests to use an explicit --packages flag.

tools/test.dart

 #!/usr/bin/env dart
 // Copyright (c) 2013, 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.
 
 /**
  * This file is the entrypoint of the dart test suite.  This suite is used
  * to test:
  *
  *     1. the dart vm
@@ skipping 38 matching lines @@
             DebugLogger.error(error);
           }
         }, onDone: completer.complete);
   } else {
     completer.complete();
   }
   return completer.future;
 }
 
 void main(List<String> arguments) {
-  // This script is in [dart]/tools.
+  // This script is in [sdk]/tools.
   TestUtils.setDartDirUri(Platform.script.resolve('..'));
   _deleteTemporaryDartDirectories().then((_) {
     var optionsParser = new TestOptionsParser();
     var configurations = optionsParser.parse(arguments);
     if (configurations != null && configurations.length > 0) {
       // All the testing is carried out asynchronously in tasks created by this
       // call.
       // TODO(26372): Ensure that all tasks complete before the returned future.
       testConfigurations(configurations);
     }
   });
 }