[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket_base.h

 // Copyright (c) 2017, 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.
 
 #ifndef RUNTIME_BIN_SOCKET_BASE_H_
 #define RUNTIME_BIN_SOCKET_BASE_H_
 
-#if defined(DART_IO_DISABLED)
-#error "socket_base.h can only be included on builds with IO enabled"
-#endif
-
 #include "platform/globals.h"
 // Declare the OS-specific types ahead of defining the generic class.
 #if defined(HOST_OS_ANDROID)
 #include "bin/socket_base_android.h"
 #elif defined(HOST_OS_FUCHSIA)
 #include "bin/socket_base_fuchsia.h"
 #elif defined(HOST_OS_LINUX)
 #include "bin/socket_base_linux.h"
 #elif defined(HOST_OS_MACOS)
 #include "bin/socket_base_macos.h"
@@ skipping 196 matching lines @@
 
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(SocketBase);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // RUNTIME_BIN_SOCKET_BASE_H_