Make test.dart override only those dependencies declared in the pubspec.yaml

tests/standalone/io/test_runner_test.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.
 
 import "dart:io";
 import "dart:isolate";
 import "dart:async";
 import "../../../tools/testing/dart/test_runner.dart";
 import "../../../tools/testing/dart/test_suite.dart";
 import "../../../tools/testing/dart/test_progress.dart" as progress;
@@ skipping 67 matching lines @@
     enqueueTestCase(testCaseTimeout);
     enqueueTestCase(testCaseFailUnexpected);
 
     if (onDone != null) {
       onDone();
     }
   }
 
   TestCase _makeNormalTestCase(name, expectations) {
     var command = CommandBuilder.instance.getProcessCommand(
-        'custom', Platform.executable, [Platform.script.toFilePath(), name],
+        'custom', Platform.executable,
+        ['--package-root=${Platform.packageRoot}', Platform.script.toFilePath(), name],
         {});
     return _makeTestCase(name, DEFAULT_TIMEOUT, command, expectations);
   }
 
   _makeCrashTestCase(name, expectations) {
     var crashCommand = CommandBuilder.instance.getProcessCommand(
         'custom_crash', getProcessTestFileName(), ["0", "0", "1", "1"],
         {});
     // The crash test sometimes times out. Run it with a large timeout
     // to help diagnose the delay.
@@ skipping 44 matching lines @@
         break;
       case 'timeout':
         // This process should be killed by the test after DEFAULT_TIMEOUT
         new Timer(new Duration(hours: 42), (){ });
         break;
       default:
         throw "Unknown option ${arguments[0]} passed to test_runner_test";
     }
   }
 }