Fix following compiler warnings:

runtime/bin/file_win.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 "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/file.h"
 
 #include <fcntl.h>  // NOLINT
@@ skipping 491 matching lines @@
 bool File::IsAbsolutePath(const char* pathname) {
   // Should we consider network paths?
   if (pathname == NULL) return false;
   return (strlen(pathname) > 2) &&
       (pathname[1] == ':') &&
       (pathname[2] == '\\' || pathname[2] == '/');
 }
 
 
 char* File::GetCanonicalPath(const char* pathname) {
-  struct _stat st;
   const wchar_t* system_name = StringUtils::Utf8ToWide(pathname);
   HANDLE file_handle = CreateFileW(
         system_name,
         0,
         FILE_SHARE_READ,
         NULL,
         OPEN_EXISTING,
         FILE_FLAG_BACKUP_SEMANTICS,
         NULL);
   if (file_handle == INVALID_HANDLE_VALUE) {
@@ skipping 123 matching lines @@
     return kIdentical;
   } else {
     return kDifferent;
   }
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)