Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/file_system_watcher_linux.cc

Index: runtime/bin/file_system_watcher_linux.cc
diff --git a/runtime/bin/file_system_watcher_linux.cc b/runtime/bin/file_system_watcher_linux.cc
index 91c81846fc07de148737fbac2bdacaa2e2f62e74..e74c69fa062932e4a23c0c8429181efa3ed75a29 100644
--- a/runtime/bin/file_system_watcher_linux.cc
+++ b/runtime/bin/file_system_watcher_linux.cc
@@ -5,7 +5,7 @@
 #if !defined(DART_IO_DISABLED)
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_LINUX)
+#if defined(HOST_OS_LINUX)
 
 #include "bin/file_system_watcher.h"
 
@@ -147,6 +147,6 @@ Dart_Handle FileSystemWatcher::ReadEvents(intptr_t id, intptr_t path_id) {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_LINUX)
+#endif  // defined(HOST_OS_LINUX)
 
 #endif  // !defined(DART_IO_DISABLED)