Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/directory_android.cc

Index: runtime/bin/directory_android.cc
diff --git a/runtime/bin/directory_android.cc b/runtime/bin/directory_android.cc
index b4ad7c04e2b4f0b3adc35aabe6cd86a36f8360b8..43ad142bbd5d2edd19357390ca3293f38ddb5c00 100644
--- a/runtime/bin/directory_android.cc
+++ b/runtime/bin/directory_android.cc
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_ANDROID)
+#if defined(HOST_OS_ANDROID)
 
 #include "bin/directory.h"
 
@@ -482,4 +482,4 @@ bool Directory::Rename(const char* path, const char* new_path) {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_ANDROID)
+#endif  // defined(HOST_OS_ANDROID)