Try to speed up output piping.

tools/testing/drt-trampoline.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.
 #
 # For now we have to use this trampoline to turn --dart-flags command line
 # switch into env variable DART_FLAGS.  Eventually, DumpRenderTree should
 # support --dart-flags and this hack may go away.
 #
 # Expected invocation: python drt-trampoline.py <path to DRT> <DRT command line>
 
@@ skipping 30 matching lines @@
         raise Exception(
             'Bad file expectation (%s), ' % out_expected_file
             + 'please specify either a .txt or a .png file')
     elif arg.endswith('.html'):
       test_file = arg
       cmd.append(arg)
     else:
       cmd.append(arg)
 
 
-  p = subprocess.Popen(cmd, env=env, stdout=subprocess.PIPE)
+  stdout = subprocess.PIPE if out_expected_file else None
+  p = subprocess.Popen(cmd, env=env, stdout=stdout)
   output, error = p.communicate()
   if p.returncode != 0:
     raise Exception('Failed to run command. return code=%s' % p.returncode)
 
   if out_expected_file:
     # Compare output to the given expectation file.
     expectation = None
     if is_png:
       # DRT prints the image to STDOUT, but includes extra text that we trim:
       # - 4 header lines
@@ skipping 16 matching lines @@
       print 'Expectation matches'
     else:
       # Generate a temporary file in the same place as the .html file:
       out_file = test_file[:test_file.rfind('.html')] + out_expected_file[-4:]
       with open(out_file, 'w') as f:
         f.write(output)
       print 'FAIL'
       print 'Expectation didn\'t match. Update expectations by running:\n'
       print 'cp %s %s\n' % (out_file, out_expected_file)
     print '#EOF'
-  else:
-    # Pipe through the output for non-layout tests.
-    print output
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv))
   except StandardError as e:
     print 'Fail: ' + str(e)
     sys.exit(1)