Add access to process id for both current process and processes started

tests/standalone/io/process_pid_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.
+//
+// Process test program to test process communication.
+
+import "package:expect/expect.dart";
+import "dart:async";
+import "dart:io";
+import "dart:isolate";
+
+main() {
+  var port = new ReceivePort();
+  Expect.isTrue(pid > 0);
+  var futures = [];
+  futures.add(Process.start(new Options().executable,['--version']));
+  futures.add(Process.run(new Options().executable,['--version']));
+  Future.wait(futures).then((results) {
+    Expect.isTrue(results[0].pid > 0);
+    Expect.isTrue(results[1].pid > 0);
+    Expect.equals(0, results[1].exitCode);
+    results[0].exitCode.then((exitCode) {
+      Expect.equals(0, exitCode);
+      port.close();
+    });
+  });
+}