Really remove io support when dart:io is unsupported.

runtime/bin/io_service_unsupported.cc

Index: runtime/bin/io_service_unsupported.cc
diff --git a/runtime/bin/io_service_unsupported.cc b/runtime/bin/io_service_unsupported.cc
index 29a8730f0dd64b71c2db0b8b13dfb3b3a5e8e7b6..b8211e701f590b449939bc02cbd6d3c92ec9c905 100644
--- a/runtime/bin/io_service_unsupported.cc
+++ b/runtime/bin/io_service_unsupported.cc
@@ -2,9 +2,10 @@
 // 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 "bin/builtin.h"
 #include "bin/dartutils.h"
-
 #include "include/dart_api.h"
 
 namespace dart {
@@ -18,3 +19,4 @@ void FUNCTION_NAME(IOService_NewServicePort)(Dart_NativeArguments args) {
 }  // namespace bin
 }  // namespace dart
 
+#endif  // defined(DART_IO_DISABLED)