Fix Windows debug assertion when cancelling pending i/o

runtime/bin/eventhandler_win.cc

 // Copyright (c) 2013, 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.
 
 #if !defined(DART_IO_DISABLED)
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/eventhandler.h"
@@ skipping 131 matching lines @@
       handle(), completion_port, reinterpret_cast<ULONG_PTR>(this), 0);
   return (completion_port_ != NULL);
 }
 
 
 void Handle::Close() {
   if (!SupportsOverlappedIO()) {
     // If the handle uses synchronous I/O (e.g. stdin), cancel any pending
     // operation before closing the handle, so the read thread is not blocked.
     BOOL result = CancelIoEx(handle_, NULL);
-    ASSERT(result);
+    ASSERT(result || (GetLastError() == ERROR_NOT_FOUND));
   }
   MonitorLocker ml(monitor_);
   if (!IsClosing()) {
     // Close the socket and set the closing state. This close method can be
     // called again if this socket has pending IO operations in flight.
     MarkClosing();
     // Perform handle type specific closing.
     DoClose();
   }
   ASSERT(IsHandleClosed());
@@ skipping 1293 matching lines @@
 void EventHandlerImplementation::Shutdown() {
   SendData(kShutdownId, 0, 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)
 
 #endif  // !defined(DART_IO_DISABLED)