Use strlen rather than strnlen, which is missing on Mac OS.

runtime/bin/secure_socket.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 "bin/secure_socket.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <stdio.h>
@@ skipping 36 matching lines @@
 
 /* Handle an error reported from the BoringSSL library. */
 static void ThrowIOException(int status,
                              const char* exception_type,
                              const char* message,
                              bool free_message = false) {
   char error_string[SSL_ERROR_MESSAGE_BUFFER_SIZE];
   error_string[0] = '\0';
   int error = ERR_get_error();
   while (error != 0) {
-    int length = strnlen(error_string, SSL_ERROR_MESSAGE_BUFFER_SIZE);
+    int length = strlen(error_string);
     int free_length = SSL_ERROR_MESSAGE_BUFFER_SIZE - length;
     if (free_length > 16) {
       // Enough room for error code at least.
       if (length > 0) {
         error_string[length] = '\n';
         error_string[length + 1] = '\0';
         length++;
         free_length--;
       }
       ERR_error_string_n(error, error_string + length, free_length);
+      // ERR_error_string_n is guaranteed to leave a null-terminated string.
     }
     error = ERR_get_error();
   }
   OSError os_error_struct(status, error_string, OSError::kBoringSSL);
   Dart_Handle os_error = DartUtils::NewDartOSError(&os_error_struct);
   Dart_Handle exception =
       DartUtils::NewDartIOException(exception_type, message, os_error);
   if (free_message) {
     free(const_cast<char*>(message));
   }
@@ skipping 1043 matching lines @@
     } else {
       if (SSL_LOG_DATA) Log::Print(
           "WriteEncrypted  BIO_read wrote %d bytes\n", bytes_processed);
     }
   }
   return bytes_processed;
 }
 
 }  // namespace bin
 }  // namespace dart