strnlen is not always available.

runtime/vm/os.h

 // 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.
 
 #ifndef VM_OS_H_
 #define VM_OS_H_
 
 #include "vm/globals.h"
 
 // Forward declarations.
@@ skipping 92 matching lines @@
   static void Sleep(int64_t millis);
 
   // Sleep the currently executing thread for micros microseconds.
   static void SleepMicros(int64_t micros);
 
   // Debug break.
   static void DebugBreak();
 
   // Not all platform support strndup.
   static char* StrNDup(const char* s, intptr_t n);
+  static intptr_t StrNLen(const char* s, intptr_t n);
 
   // Print formatted output to stdout/stderr for debugging.
   static void Print(const char* format, ...) PRINTF_ATTRIBUTE(1, 2);
   static void PrintErr(const char* format, ...) PRINTF_ATTRIBUTE(1, 2);
   static void VFPrint(FILE* stream, const char* format, va_list args);
   // Print formatted output info a buffer.
   //
   // Does not write more than size characters (including the trailing '\0').
   //
   // Returns the number of characters (excluding the trailing '\0')
@@ skipping 39 matching lines @@
   static void Shutdown();
 
   static void Abort();
 
   static void Exit(int code);
 };
 
 }  // namespace dart
 
 #endif  // VM_OS_H_