Update dart:io to convert strings between UTF8 and current code page

runtime/bin/utils_macos.cc

Index: runtime/bin/utils_macos.cc
diff --git a/runtime/bin/utils_macos.cc b/runtime/bin/utils_macos.cc
index f364e475700fc02c3e01b21471720b03858b6afb..ab48f555e9d9856a3b30b993600f6f14f809e857 100644
--- a/runtime/bin/utils_macos.cc
+++ b/runtime/bin/utils_macos.cc
@@ -26,3 +26,19 @@ void OSError::SetCodeAndMessage(SubSystem sub_system, int code) {
     UNREACHABLE();
   }
 }
+
+const char* StringUtils::SystemStringToUtf8(const char* str) {
+  return str;
+}
+
+const char* StringUtils::Utf8ToSystemString(const char* utf8) {
+  return utf8;
+}
+
+char* StringUtils::SystemStringToUtf8(char* str) {
+  return str;
+}
+
+char* StringUtils::Utf8ToSystemString(char* utf8) {
+  return utf8;
+}