Use a marker instead of a string so it can be distingished when used as stdin.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/6840053

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@81941 0039d316-1c4b-4281-b951-d872f2087c98

subprocess2.py

@@ -21,7 +21,7 @@ import threading
 PIPE = subprocess.PIPE
 STDOUT = subprocess.STDOUT
 # Sends stdout or stderr to os.devnull.
-VOID = '/dev/null'
+VOID = object()
 # Error code when a process was killed because it timed out.
 TIMED_OUT = -2001
 
@@ -203,7 +203,11 @@ def call(args, timeout=None, **kwargs):
   """
   stdin = kwargs.pop('stdin', None)
   if stdin is not None:
-    assert stdin != PIPE
+    if stdin is VOID:
+      kwargs['stdin'] = open(os.devnull, 'r')
+      stdin = None
+    else:
+      assert isinstance(stdin, str)
       # When stdin is passed as an argument, use it as the actual input data and
       # set the Popen() parameter accordingly.
       kwargs['stdin'] = PIPE