[dart:io] Remove DART_IO_DISABLED

runtime/bin/stdio_linux.cc

 // Copyright (c) 2013, 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 "platform/globals.h"
 #if defined(HOST_OS_LINUX)
 
 #include "bin/stdio.h"
 
 #include <errno.h>      // NOLINT
 #include <sys/ioctl.h>  // NOLINT
 #include <termios.h>    // NOLINT
 
 #include "bin/fdutils.h"
@@ skipping 87 matching lines @@
 
 bool Stdout::AnsiSupported(intptr_t fd, bool* supported) {
   *supported = isatty(fd) && TermHasXTerm();
   return true;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)