Update test.dart to do the same on all platforms for tests run

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index 994110e65ebc076b0d46ead89780e4538b3134f3..4d8917b3dfa2e99ea71a636271e7b55686b6f71e 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -750,19 +750,21 @@ class BatchRunnerProcess {
   Future terminate() {
     if (_process == null) return new Future.immediate(true);
     Completer completer = new Completer();
+    Timer killTimer;
     _process.onExit = (exitCode) {
       _process.close();
+      if (killTimer != null) killTimer.cancel();
       completer.complete(true);
     };
-    if (_isWebDriver && Platform.operatingSystem == 'windows') {
+    if (_isWebDriver) {
       // Use a graceful shutdown so our Selenium script can close
-      // the open browser processes. TODO(jmesserly): Send a signal once
-      // that's supported in Windows.
+      // the open browser processes. On Windows, signals do not exist

[Jennifer Messerly, 2012/10/09 17:54:56]

I'm confused by this comment. See:
http://msdn.microsoft.com/en-us/library/dwwzkt4c(v=vs.100).aspx
and http://msdn.microsoft.com/en-us/library/xdkz3x12(v=vs.100).aspx

Granted, I haven't tried this in a long time, but IIRC Python on Windows has
basic signal support.

[Emily Fortuna, 2012/10/09 18:01:41]

It does
http://docs.python.org/library/subprocess.html#subprocess.Popen.send_signal (and
also the Python win32 API)

Dart just has to send the right information when closing the process, which it
is currently not doing: http://code.google.com/p/dart/source/detail?r=8541

+      // and a kill is a hard kill.
       _process.stdin.write('--terminate\n'.charCodes());
 
       // In case the run_selenium process didn't close, kill it after 30s
       int shutdownMillisecs = 30000;
-      new Timer(shutdownMillisecs, (e) { if (!closed) _process.kill(); });
+      killTimer = new Timer(shutdownMillisecs, (e) { _process.kill(); });
     } else {
       _process.kill();
     }