Update test scripts to deal with current VM behavior on optional constructor arguments.

tests/standalone/src/ProcessTestUtil.dart

 // Copyright (c) 2011, 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.
 
 String getProcessTestFileName() {
-  var names = ['out/Release_ia32/process_test',
-               'out/Debug_ia32/process_test',
-               'xcodebuild/Release_ia32/process_test',
-               'xcodebuild/Debug_ia32/process_test',
-               'Release_ia32/process_test.exe',
-               'Debug_ia32/process_test.exe'];
+  var os = new Platform().operatingSystem();
+
+  var outDir = '';
+  if (os == 'linux') {
+    outDir = 'out';
+  } else if (os == 'macos') {
+    outDir = 'xcodebuild';
+  }
+
+  var names = ['$outDir/Release_ia32/process_test',
+               '$outDir/Debug_ia32/process_test'];
+
   for (var name in names) {
     if (new File(name).existsSync()) {
       return name;
     }
   }
+
   Expect.fail('Could not find the process_test program.');
 }