Fixes leak of native File objects.

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 20 matching lines @@
   void set_fd(int fd) { fd_ = fd; }
 
  private:
   int fd_;
 
   DISALLOW_COPY_AND_ASSIGN(FileHandle);
 };
 
 
 File::~File() {
-  Close();
+  if (!IsClosed()) {
+    Close();
+  }
   delete handle_;
 }
 
 
 void File::Close() {
   ASSERT(handle_->fd() >= 0);
   if ((handle_->fd() == _fileno(stdout)) ||
       (handle_->fd() == _fileno(stderr))) {
     int fd = _open("NUL", _O_WRONLY);
     ASSERT(fd >= 0);
@@ skipping 633 matching lines @@
     return kIdentical;
   } else {
     return kDifferent;
   }
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)