Process exit code handling reworked for Windows.

tests/standalone/io/process_many_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:io');
+
+endProcesses(processes) {
+  processes.forEach((p) {
+    p.stdin.writeString("line\n");
+    p.onExit = (code) => Expect.equals(0, code);
+  });
+}
+
+main() {
+  var num = 75;
+  var started = 0;
+  var processes = [];
+  for (var i = 0; i < num; i++) {
+    var script = new File("tests/standalone/io/process_many_script.dart");
+    if (!script.existsSync()) {
+      script = new File("../tests/standalone/io/process_many_script.dart");
+    }
+    var p = Process.start(new Options().executable, [script.name]);
+    processes.add(p);
+    p.onStart = () {
+      if (++started == num) endProcesses(processes);
+    };
+  }
+}