Use a sync-writing StreamConsumer for stdout/stderr.

runtime/bin/stdio_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class _StdIOUtils {
   static Stdin _getStdioInputStream() {
     switch (_getStdioHandleType(0)) {
       case _STDIO_HANDLE_TYPE_TERMINAL:
       case _STDIO_HANDLE_TYPE_PIPE:
       case _STDIO_HANDLE_TYPE_SOCKET:
@@ skipping 12 matching lines @@
       } else {
         return new _StdSink(sink);
       }
     }
     assert(fd == 1 || fd == 2);
     switch (_getStdioHandleType(fd)) {
       case _STDIO_HANDLE_TYPE_TERMINAL:
       case _STDIO_HANDLE_TYPE_PIPE:
       case _STDIO_HANDLE_TYPE_SOCKET:
       case _STDIO_HANDLE_TYPE_FILE:
-        return wrap(new IOSink(new _FileStreamConsumer.fromStdio(fd)));
+        return wrap(new IOSink(new _StdConsumer(fd)));
       default:
         throw new FileSystemException("Unsupported stdin type");
     }
   }
 
   static int _socketType(nativeSocket) {
     var result = _getSocketType(nativeSocket);
     if (result is OSError) {
       throw new FileSystemException("Error retreiving socket type", result);
     }
@@ skipping 38 matching lines @@
     }
     return size;
   }
 
   static _getTerminalSize() native "Stdout_GetTerminalSize";
 }
 
 
 _getStdioHandle(_NativeSocket socket, int num) native "Socket_GetStdioHandle";
 _getSocketType(_NativeSocket nativeSocket) native "Socket_GetType";