Use native fields for process pid and fix bug that allowed killing

tests/standalone/io/process_kill_unstarted_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.
+//
+// Process test program to test that an unstarted process cannot be killed.
+
+#library("ProcessKillUnstartedTest");
+#import("dart:io");
+
+main() {
+  var p = Process.start('________', []);
+  Expect.throws(p.kill, (e) => e is ProcessException);
+  p.onError = (e) => Expect.isTrue(e is ProcessException);
+  p.onStart = () => Expect.fail("Process not expected to start");
+}