Don't use same value for socket() and handle(), as they are different concepts.

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.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/eventhandler.h"
 
 #include <winsock2.h>  // NOLINT
@@ skipping 141 matching lines @@
 void Handle::Lock() {
   EnterCriticalSection(&cs_);
 }
 
 
 void Handle::Unlock() {
   LeaveCriticalSection(&cs_);
 }
 
 
-bool Handle::CreateCompletionPort(HANDLE completion_port) {
-  completion_port_ = CreateIoCompletionPort(handle_,
+bool Handle::DoCreateCompletionPort(HANDLE handle, HANDLE completion_port) {
+  completion_port_ = CreateIoCompletionPort(handle,
                                             completion_port,
                                             reinterpret_cast<ULONG_PTR>(this),
                                             0);
   if (completion_port_ == NULL) {
     return false;
   }
   return true;
 }
 
 
+bool Handle::CreateCompletionPort(HANDLE completion_port) {
+  return DoCreateCompletionPort(handle_, completion_port_);
+}
+
+
 void Handle::Close() {
   ScopedLock lock(this);
   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 241 matching lines @@
 
   if (pending_read_ != NULL) {
     CancelIoEx(handle(), pending_read_->GetCleanOverlapped());
     // Don't dispose of the buffer, as it will still complete (with length 0).
   }
 
   DoClose();
 }
 
 
+bool SocketHandle::CreateCompletionPort(HANDLE completion_port) {
+  HANDLE handle = reinterpret_cast<HANDLE>(socket());
+  return DoCreateCompletionPort(handle, completion_port);
+}
+
+
 void SocketHandle::HandleIssueError() {
   int error = WSAGetLastError();
   if (error == WSAECONNRESET) {
     HandleClosed(this);
   } else {
     HandleError(this);
   }
   WSASetLastError(error);
 }
 
@@ skipping 918 matching lines @@
 
 
 void EventHandlerImplementation::Shutdown() {
   SendData(kShutdownId, 0, 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)