Use output-only string streams instead of input-output string streams.

runtime/platform/assert.cc

 // Copyright (c) 2012, 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 "platform/assert.h"
 
 #include <cstdlib>
 #include <sstream>
 #include <string>
 
 #include "platform/globals.h"
 
 namespace dart {
 
 // Exit with a failure code when we miss an EXPECT check.
 static void failed_exit(void) {
   exit(255);
 }
 
 void DynamicAssertionHelper::Fail(const char* format, ...) {
-  std::stringstream stream;
+  std::ostringstream stream;
   stream << file_ << ":" << line_ << ": error: ";
 
   va_list arguments;
   va_start(arguments, format);
   char buffer[2 * KB];
   vsnprintf(buffer, sizeof(buffer), format, arguments);
   va_end(arguments);
   stream << buffer << std::endl;
 
   // Get the message from the string stream and dump it on stderr.
   std::string message = stream.str();
   fprintf(stderr, "%s", message.c_str());
 
   // In case of failed assertions, abort right away. Otherwise, wait
   // until the program is exiting before producing a non-zero exit
   // code through abort.
   // TODO(5411324): replace std::abort with OS::Abort so that we can handle
   // restoring of signal handlers before aborting.
   if (kind_ == ASSERT) {
     std::abort();
   }
   static bool failed = false;
   if (!failed) {
     std::atexit(&failed_exit);
   }
   failed = true;
 }
 
 }  // namespace dart