Make standalone tests not dependent on current working directory.

tests/standalone/io/platform_test.dart

 // Copyright (c) 2012, 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:io";
 import "dart:isolate";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
@@ skipping 49 matching lines @@
     Expect.equals("file", uri.scheme);
     Expect.isTrue(uri.path.endsWith('tests/standalone/io/platform_test.dart'));
     Expect.equals(Platform.packageRoot, results["Platform.packageRoot"]);
     Expect.listEquals(Platform.executableArguments,
                       results["Platform.executableArguments"]);
     asyncEnd();
   });
 }
 
 main() {
+  // This tests assumes paths relative to dart main directory
+  Directory.current = Platform.script.resolve('../../..').toFilePath();
   test();
   testIsolate();
 }