Fix handling of async IO from pipes on Windows

runtime/bin/process_test.cc

 // 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.
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
 /*
  * Run ./process_test <outstream> <echocount> <exitcode> <crash>
@@ skipping 20 matching lines @@
   int exit_code = atoi(argv[3]);
   int crash = atoi(argv[4]);
 
   const int kLineSize = 128;
   char line[kLineSize];
 
   while ((echo_count != echo_counter) &&
          (fgets(line, kLineSize, stdin) != NULL)) {
     if (outstream == 0) {
       fprintf(stdout, "%s", line);
+      fflush(stdout);
     } else if (outstream == 1) {
       fprintf(stderr, "%s", line);
+      fflush(stderr);
     } else if (outstream == 2) {
       fprintf(stdout, "%s", line);
       fprintf(stderr, "%s", line);
+      fflush(stdout);
+      fflush(stderr);
     }
     echo_counter++;
   }
 
   if (crash == 1) {
     int* segfault = NULL;
     *segfault = 1;
   }
 
-
   return exit_code;
 }