Really remove io support when dart:io is unsupported.

runtime/bin/process_macos.cc

Index: runtime/bin/process_macos.cc
diff --git a/runtime/bin/process_macos.cc b/runtime/bin/process_macos.cc
index 00b2d8ef3bbdb1260e783b70e367bb79a4e67af8..b3b5662474f2801823439cab1c0ef490c76255be 100644
--- a/runtime/bin/process_macos.cc
+++ b/runtime/bin/process_macos.cc
@@ -2,6 +2,8 @@
 // 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(TARGET_OS_MACOS)
 
@@ -31,6 +33,9 @@
 namespace dart {
 namespace bin {
 
+int Process::global_exit_code_ = 0;
+Mutex* Process::global_exit_code_mutex_ = new Mutex();
+
 // ProcessInfo is used to map a process id to the file descriptor for
 // the pipe used to communicate the exit code of the process to Dart.
 // ProcessInfo objects are kept in the static singly-linked
@@ -1104,3 +1109,5 @@ void Process::ClearSignalHandler(intptr_t signal) {
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)
+
+#endif  // !defined(DART_IO_DISABLED)