Use MacOS helper to get address of environ variable

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 a6d9301b421cd5f83e0224e7135908aa873f9d50..3df775f568c933aa4a0d99a9d5a610083fdac2be 100644
--- a/runtime/bin/process_macos.cc
+++ b/runtime/bin/process_macos.cc
@@ -7,6 +7,7 @@
 
 #include "bin/process.h"
 
+#include <crt_externs.h>  // NOLINT
 #include <errno.h>  // NOLINT
 #include <fcntl.h>  // NOLINT
 #include <poll.h>  // NOLINT
@@ -24,8 +25,6 @@
 #include "platform/signal_blocker.h"
 
 
-extern char **environ;
-
 
 namespace dart {
 namespace bin {
@@ -474,6 +473,9 @@ int Process::Start(const char* path,
       }
 
       if (program_environment != NULL) {
+        // On MacOS you have to do a bit of magic to get to the
+        // environment strings.
+        char** environ = *(_NSGetEnviron());
         environ = program_environment;
       }