[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket_base.cc

Index: runtime/bin/socket_base.cc
diff --git a/runtime/bin/socket_base.cc b/runtime/bin/socket_base.cc
index 879f5b90f746f319da5b1537f49bac30ee69aa8a..060c71063faba363262eac4fb8d08612ca9a7887 100644
--- a/runtime/bin/socket_base.cc
+++ b/runtime/bin/socket_base.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_base.h"
 
 #include "bin/dartutils.h"
@@ -179,5 +177,3 @@ void FUNCTION_NAME(SocketBase_IsBindError)(Dart_NativeArguments args) {
 
 }  // namespace bin
 }  // namespace dart
-
-#endif  // !defined(DART_IO_DISABLED)