Introduce a kSetEventMaskCommand, some cleanups in windows eventhandler

dart/runtime/bin/eventhandler_linux.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_LINUX)
 
 #include "bin/eventhandler.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 201 matching lines @@
           sd->Close();
           socket_map_.Remove(GetHashmapKeyFromFd(fd), GetHashmapHashFromFd(fd));
           delete sd;
         }
         DartUtils::PostInt32(msg[i].dart_port, 1 << kDestroyedEvent);
       } else if (IS_COMMAND(msg[i].data, kReturnTokenCommand)) {
         int count = TOKEN_COUNT(msg[i].data);
         if (sd->ReturnToken(msg[i].dart_port, count)) {
           AddToEpollInstance(epoll_fd_, sd);
         }
-      } else {
-        ASSERT_NO_COMMAND(msg[i].data);
+      } else if (IS_COMMAND(msg[i].data, kSetEventMaskCommand)) {
+        // `events` can only have kInEvent/kOutEvent flags set.
+        intptr_t events = msg[i].data & EVENT_MASK;
+        ASSERT(0 == (events & ~(1 << kInEvent) & ~(1 << kOutEvent)));
+
         // Setup events to wait for.
         if (sd->AddPort(msg[i].dart_port)) {
-          sd->SetMask(msg[i].data);
+          sd->SetMask(events);
           AddToEpollInstance(epoll_fd_, sd);
         }
+      } else {
+        UNREACHABLE();
       }
     }
   }
 }
 
 #ifdef DEBUG_POLL
 static void PrintEventMask(intptr_t fd, intptr_t events) {
   Log::Print("%d ", fd);
   if ((events & EPOLLIN) != 0) Log::Print("EPOLLIN ");
   if ((events & EPOLLPRI) != 0) Log::Print("EPOLLPRI ");
@@ skipping 118 matching lines @@
 
 uint32_t EventHandlerImplementation::GetHashmapHashFromFd(intptr_t fd) {
   // The hashmap does not support keys with value 0.
   return dart::Utils::WordHash(fd + 1);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)