Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/directory_fuchsia.cc

Index: runtime/bin/directory_fuchsia.cc
diff --git a/runtime/bin/directory_fuchsia.cc b/runtime/bin/directory_fuchsia.cc
index f47f81d49d4805dcc549ba9308b597db9a3f85a1..c2139104c7e6aefb1438ebe4381ce8de43b4539f 100644
--- a/runtime/bin/directory_fuchsia.cc
+++ b/runtime/bin/directory_fuchsia.cc
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_FUCHSIA)
+#if defined(HOST_OS_FUCHSIA)
 
 #include "bin/directory.h"
 
@@ -431,4 +431,4 @@ bool Directory::Rename(const char* path, const char* new_path) {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)