Really remove io support when dart:io is unsupported.

runtime/bin/eventhandler_unsupported.cc

+// Copyright (c) 2016, 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.
+
+#if defined(DART_IO_DISABLED)
+
+#include "bin/eventhandler.h"
+
+#include "bin/builtin.h"
+#include "bin/dartutils.h"
+#include "include/dart_api.h"
+
+namespace dart {
+namespace bin {
+
+void EventHandler::Start() {
+}
+
+
+void EventHandler::Stop() {
+}
+
+
+void FUNCTION_NAME(EventHandler_SendData)(Dart_NativeArguments args) {
+  Dart_ThrowException(DartUtils::NewInternalError(
+        "EventHandler is not supported on this platform"));
+}
+
+
+void FUNCTION_NAME(EventHandler_TimerMillisecondClock)(
+    Dart_NativeArguments args) {
+  Dart_ThrowException(DartUtils::NewInternalError(
+        "EventHandler is not supported on this platform"));
+}
+
+}  // namespace bin
+}  // namespace dart
+
+#endif  // defined(DART_IO_DISABLED)