Really remove io support when dart:io is unsupported.

runtime/bin/socket.cc

Index: runtime/bin/socket.cc
diff --git a/runtime/bin/socket.cc b/runtime/bin/socket.cc
index cba96c0ecfd189f498010d9a3f582b3c9ab1fad2..fb9704b41d95224575e753f23f9154174153fed9 100644
--- a/runtime/bin/socket.cc
+++ b/runtime/bin/socket.cc
@@ -2,19 +2,22 @@
 // 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/socket.h"
+
 #include "bin/dartutils.h"
 #include "bin/io_buffer.h"
 #include "bin/isolate_data.h"
 #include "bin/lockers.h"
-#include "bin/socket.h"
 #include "bin/thread.h"
 #include "bin/utils.h"
 
+#include "include/dart_api.h"
+
 #include "platform/globals.h"
 #include "platform/utils.h"
 
-#include "include/dart_api.h"
-
 namespace dart {
 namespace bin {
 
@@ -874,3 +877,5 @@ intptr_t Socket::GetSocketIdNativeField(Dart_Handle socket_obj) {
 
 }  // namespace bin
 }  // namespace dart
+
+#endif  // !defined(DART_IO_DISABLED)