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

runtime/bin/platform_macos.cc

Index: runtime/bin/platform_macos.cc
diff --git a/runtime/bin/platform_macos.cc b/runtime/bin/platform_macos.cc
index c5759d8c427f344aca99aeb92afa5ab0e5428a4c..822399d7328ce1fafd80eb61b423c10854c8f31c 100644
--- a/runtime/bin/platform_macos.cc
+++ b/runtime/bin/platform_macos.cc
@@ -67,6 +67,11 @@ char** Platform::Environment(intptr_t* count) {
 }
 
 
+void Platform::FreeEnvironment(char** env, intptr_t count) {
+  delete[] env;
+}
+
+
 char* Platform::StrError(int error_code) {
   static const int kBufferSize = 1024;
   char* error = static_cast<char*>(malloc(kBufferSize));