strnlen is not always available.

runtime/vm/os_linux.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 "vm/globals.h"
 #if defined(TARGET_OS_LINUX)
 
 #include "vm/os.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 282 matching lines @@
 void OS::DebugBreak() {
   __builtin_trap();
 }
 
 
 char* OS::StrNDup(const char* s, intptr_t n) {
   return strndup(s, n);
 }
 
 
+intptr_t OS::StrNLen(const char* s, intptr_t n) {
+  return strnlen(s, n);
+}
+
+
 void OS::Print(const char* format, ...) {
   va_list args;
   va_start(args, format);
   VFPrint(stdout, format, args);
   va_end(args);
 }
 
 
 void OS::VFPrint(FILE* stream, const char* format, va_list args) {
   vfprintf(stream, format, args);
@@ skipping 107 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)