[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket.cc

Index: runtime/bin/socket.cc
diff --git a/runtime/bin/socket.cc b/runtime/bin/socket.cc
index 3f4ac0332224928cc53b36db40561a72d9222404..e2e16778c3fb6179976d06a4e5ef4a384fbd1b0f 100644
--- a/runtime/bin/socket.cc
+++ b/runtime/bin/socket.cc
@@ -2,8 +2,6 @@
 // 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"
@@ -967,5 +965,3 @@ Socket* Socket::GetSocketIdNativeField(Dart_Handle socket_obj) {
 
 }  // namespace bin
 }  // namespace dart
-
-#endif  // !defined(DART_IO_DISABLED)