Use strict equality when comparing with null, especially when null is a default value.

runtime/bin/process_impl.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.
 
 class _ProcessStartStatus {
   int _errorCode;  // Set to OS error code if process start failed.
   String _errorMessage;  // Set to OS error message if process start failed.
 }
 
 
@@ skipping 96 matching lines @@
   }
 
   OutputStream get stdin() {
     if (_closed) {
       throw new ProcessException("Process closed");
     }
     return _out.outputStream;
   }
 
   bool kill() {
-    if (_closed && _pid == null) {
+    if (_closed && _pid === null) {
       throw new ProcessException("Process closed");
     }
     if (_killed) {
       return true;
     }
     if (_kill(_pid)) {
       _killed = true;
       return true;
     }
     return false;
@@ skipping 27 matching lines @@
   Socket _in;
   Socket _out;
   Socket _err;
   Socket _exitHandler;
   int _pid;
   bool _closed;
   bool _killed;
   bool _started;
   var _exitHandlerCallback;
 }