This works fine:

`tools/swarming_client/swarming.py trigger -S https://chromium-swarm.appspot.com -I https://isolateserver.appspot.com -d pool Chrome -s 5c98c21812a49406ebb7c23be51656188916554d -d os Windows -- --gtest_filter=ProxyResolverV8Test.SideEffects` => https://chromium-swarm.appspot.com/task?id=3be8760b192b5110&refresh=10&show_raw=1
[1/1] ProxyResolverV8Test.SideEffects (127 ms)
SUCCESS: all tests passed.



If I add --dump-json, the run fails:

`tools/swarming_client/swarming.py trigger -S https://chromium-swarm.appspot.com -I https://isolateserver.appspot.com -d pool Chrome -s 5c98c21812a49406ebb7c23be51656188916554d --dump-json results/0.json -d os Windows -- --gtest_filter=ProxyResolverV8Test.SideEffects` => https://chromium-swarm.appspot.com/task?id=3be8750ff8ef0a10&refresh=10&show_raw=1
Traceback (most recent call last):
  File "..\..\testing\test_env.py", line 288, in <module>
    sys.exit(main())
  File "..\..\testing\test_env.py", line 284, in main
    return run_executable(sys.argv[1:], os.environ.copy())
  File "..\..\testing\test_env.py", line 248, in run_executable
    '-vpython-tool', 'install',
  File "c:\infra-system\bin\lib\subprocess.py", line 181, in check_call
    retcode = call(*popenargs, **kwargs)
  File "c:\infra-system\bin\lib\subprocess.py", line 168, in call
    return Popen(*popenargs, **kwargs).wait()
  File "c:\infra-system\bin\lib\subprocess.py", line 390, in __init__
    errread, errwrite)
  File "c:\infra-system\bin\lib\subprocess.py", line 640, in _execute_child
    startupinfo)
WindowsError: [Error 2] The system cannot find the file specified