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

runtime/bin/process_impl.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.
 
 _exit(int status) native "Exit";
 
 class _ProcessStartStatus {
   int _errorCode;  // Set to OS error code if process start failed.
   String _errorMessage;  // Set to OS error message if process start failed.
 }
 
 
-class _Process extends Process {
+class _Process extends NativeFieldWrapperClass1 implements Process {
   static Future<ProcessResult> run(String path,
                                    List<String> arguments,
                                    [ProcessOptions options]) {
     return new _NonInteractiveProcess._start(path, arguments, options)._result;
   }
 
   _Process.start(String path,
                  List<String> arguments,
                  ProcessOptions options) {
     if (path is !String) {
@@ skipping 196 matching lines @@
       throw new ProcessException("Process closed");
     }
     return _out.outputStream;
   }
 
   void kill([ProcessSignal signal = ProcessSignal.SIGTERM]) {
     if (signal is! ProcessSignal) {
       throw new IllegalArgumentException(
           "Argument 'signal' must be a ProcessSignal");
     }
-    if (_closed && _pid === null) {
-      _reportError(new ProcessException("Process closed"));
+    if (!_started) {
+      var e = new ProcessException("Cannot kill process that is not started");
+      _reportError(e);
       return;
     }
     if (_ended) {
       return;
     }
-    // TODO(ager): Make the actual kill operation asynchronous.
-    if (_kill(_pid, signal._signalNumber)) {
+    if (_kill(this, signal._signalNumber)) {
       return;
     }
     _reportError(new ProcessException("Could not kill process"));
     return;
   }
 
-  bool _kill(int pid, int signal) native "Process_Kill";
+  bool _kill(Process p, int signal) native "Process_Kill";
 
   void close() {
     if (_closed) {
       throw new ProcessException("Process closed");
     }
     _in.close();
     _out.close();
     _err.close();
     _exitHandler.close();
     _closed = true;
@@ skipping 27 matching lines @@
 
   String _path;
   ObjectArray<String> _arguments;
   String _workingDirectory;
   List<String> _environment;
   // Private methods of _Socket are used by _in, _out, and _err.
   _Socket _in;
   _Socket _out;
   _Socket _err;
   Socket _exitHandler;
-  int _pid;
   bool _closed;
   bool _ended;
   bool _started;
   Function _onExit;
   Function _onError;
   Function _onStart;
 }
 
 
 // _NonInteractiveProcess is a wrapper around an interactive process
@@ skipping 87 matching lines @@
 
 class _ProcessResult implements ProcessResult {
   const _ProcessResult(int this.exitCode,
                        String this.stdout,
                        String this.stderr);
 
   final int exitCode;
   final String stdout;
   final String stderr;
 }