Fixes some memory leaks in //runtime/bin

runtime/bin/utils_win.h

 // 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.
 
 #ifndef BIN_UTILS_WIN_H_
 #define BIN_UTILS_WIN_H_
 
 #include "platform/globals.h"
 
 namespace dart {
 namespace bin {
 
 void FormatMessageIntoBuffer(DWORD code, wchar_t* buffer, int buffer_length);
 
+// These string utility functions return strings that have been allocated with
+// Dart_ScopeAllocate(). They should be used only when we are inside an API
+// scope. If a string returned by one of these functions must persist beyond
+// the scope, then copy the results into a suitable buffer that you have
+// allocated.
 class StringUtilsWin {
  public:
   static char* WideToUtf8(wchar_t* wide,
                           intptr_t len = -1,
                           intptr_t* result_len = NULL);
   static const char* WideToUtf8(const wchar_t* wide,
                                 intptr_t len = -1,
                                 intptr_t* result_len = NULL);
   static wchar_t* Utf8ToWide(char* utf8,
                              intptr_t len = -1,
                              intptr_t* result_len = NULL);
   static const wchar_t* Utf8ToWide(const char* utf8,
                                    intptr_t len = -1,
                                    intptr_t* result_len = NULL);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_UTILS_WIN_H_