Reapply directory error handling change.

runtime/bin/platform.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 BIN_PLATFORM_H_
 #define BIN_PLATFORM_H_
 
 #include "bin/builtin.h"
 
 class Platform {
  public:
+  // Returns the number of processors on the machine.
   static int NumberOfProcessors();
+
+  // Returns a string representing the operating system ("linux",
+  // "macos" or "windows"). The returned string should not be
+  // deallocated by the caller.
   static const char* OperatingSystem();
 
+  // Returns a string representation of an error code. The returned
+  // string must be deallocated by the caller.
+  static char* StrError(int error_code);
+
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Platform);
 };
 
 #endif  // BIN_PLATFORM_H_