Only use one process when running tests on Windows

tools/testing/test_runner.py

 # 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.
 #
 """Classes and methods for executing tasks for the test.py framework.
 
 This module includes:
   - Managing parallel execution of tests using threads
   - Windows and Unix specific code for spawning tasks and retrieving results
   - Evaluating the output of each test as  pass/fail/crash/timeout
@@ skipping 207 matching lines @@
   """Implements communication with a set of subprocesses using threads."""
 
   def __init__(self, work_queue, tasks, progress, batch_cmd):
     super(BatchRunner, self).__init__(work_queue, tasks, progress)
     self.runners = {}
     self.last_activity = {}
     self.context = progress.context
 
     # Scale the number of tasks to the nubmer of CPUs on the machine
     # 1:1 is too much of an overload on many machines in batch mode,
-    # so scale the ratio of threads to CPUs back.
+    # so scale the ratio of threads to CPUs back. On Windows running
+    # more than one task is not safe.
     if tasks == testing.USE_DEFAULT_CPUS:
-      tasks = .75 * testing.HOST_CPUS
+      if utils.IsWindows():
+        tasks = 1
+      else:
+        tasks = .75 * testing.HOST_CPUS
 
     # Start threads
     for i in xrange(tasks):
       thread = threading.Thread(target=self.RunThread, args=[batch_cmd, i])
       self.threads.append(thread)
       thread.daemon = True
       thread.start()
 
   def RunThread(self, batch_cmd, thread_number):
     """A thread started to feed a single TestRunner."""
@@ skipping 173 matching lines @@
   def Shutdown(self):
     """Kill all active runners."""
     print 'Shutting down remaining runners.'
     self.terminate = True
     for runner in self.runners.values():
       runner.kill()
     # Give threads a chance to exit gracefully
     time.sleep(2)
     for runner in self.runners.values():
       self.EndRunner(runner)