Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/thread_fuchsia.cc

Index: runtime/bin/thread_fuchsia.cc
diff --git a/runtime/bin/thread_fuchsia.cc b/runtime/bin/thread_fuchsia.cc
index c4fb413d9fced6d9adf70f1d920bc6b4ac72bdc2..5a9b35103af78bad7c7cc95655928cbb3e2418bb 100644
--- a/runtime/bin/thread_fuchsia.cc
+++ b/runtime/bin/thread_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/thread.h"
 #include "bin/thread_fuchsia.h"
@@ -321,4 +321,4 @@ void Monitor::NotifyAll() {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)