Introduce a kSetEventMaskCommand, some cleanups in windows eventhandler

dart/runtime/bin/eventhandler_linux.cc

Index: dart/runtime/bin/eventhandler_linux.cc
diff --git a/dart/runtime/bin/eventhandler_linux.cc b/dart/runtime/bin/eventhandler_linux.cc
index 7f81e3aa80e09e49cf60c6e6d5e623f6acdc0c45..d3ccb8d2133841612211e72d28f37e6e6dd38a11 100644
--- a/dart/runtime/bin/eventhandler_linux.cc
+++ b/dart/runtime/bin/eventhandler_linux.cc
@@ -219,13 +219,18 @@ void EventHandlerImplementation::HandleInterruptFd() {
         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();
       }
     }
   }