Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/directory_linux.cc

Index: runtime/bin/directory_linux.cc
diff --git a/runtime/bin/directory_linux.cc b/runtime/bin/directory_linux.cc
index f2a064aeca8c8e8c452c920a969f4b23bf3c9f2e..b3acd6ff11992f8925d7b665e1ac9429ba7e3bc3 100644
--- a/runtime/bin/directory_linux.cc
+++ b/runtime/bin/directory_linux.cc
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_LINUX)
+#if defined(HOST_OS_LINUX)
 
 #include "bin/directory.h"
 
@@ -486,4 +486,4 @@ bool Directory::Rename(const char* path, const char* new_path) {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_LINUX)
+#endif  // defined(HOST_OS_LINUX)