Really remove io support when dart:io is unsupported.

runtime/bin/eventhandler_win.cc

Index: runtime/bin/eventhandler_win.cc
diff --git a/runtime/bin/eventhandler_win.cc b/runtime/bin/eventhandler_win.cc
index 26982534c36bfde91f2232a9b6d6e87ac88d65b1..dfd4985a8e3bfc160716b917a6cd709f70c066c7 100644
--- a/runtime/bin/eventhandler_win.cc
+++ b/runtime/bin/eventhandler_win.cc
@@ -2,6 +2,8 @@
 // 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)
 
@@ -1505,3 +1507,5 @@ void EventHandlerImplementation::Shutdown() {
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)
+
+#endif  // !defined(DART_IO_DISABLED)