Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/directory_macos.cc

Index: runtime/bin/directory_macos.cc
diff --git a/runtime/bin/directory_macos.cc b/runtime/bin/directory_macos.cc
index ca0d41bb1449723a0fadbcf845ae61872b1c5dc3..48a7088dc9765f154ab3c6416221a232a9e34eea 100644
--- a/runtime/bin/directory_macos.cc
+++ b/runtime/bin/directory_macos.cc
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_MACOS)
+#if defined(HOST_OS_MACOS)
 
 #include "bin/directory.h"
 
@@ -477,4 +477,4 @@ bool Directory::Rename(const char* path, const char* new_path) {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_MACOS)
+#endif  // defined(HOST_OS_MACOS)