Redirect stderr to sys.stdout for build output.

The stderr is currently not being displayed on the buildbot.
Review URL: https://codereview.chromium.org//11228050

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@13932 260f80e4-7a28-3924-810f-c04153c831b5

tools/build.py

@@ -316,7 +316,9 @@ def Main():
               print k + " = " + v
 
         print ' '.join(args)
-        process = subprocess.Popen(args)
+        # We redirect stderr to sys.stdout bacause the buildbot is not
+        # collecting stderr output.
+        process = subprocess.Popen(args, stderr=sys.stdout)
         process.wait()
         if process.returncode != 0:
           print "BUILD FAILED"