Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/file_fuchsia.cc

Index: runtime/bin/file_fuchsia.cc
diff --git a/runtime/bin/file_fuchsia.cc b/runtime/bin/file_fuchsia.cc
index 5ed46d503361fc1b5ace624c8aacea1122f052dc..e0d62b41857c2b3ef35bdcb2e18eddfc98c35b6d 100644
--- a/runtime/bin/file_fuchsia.cc
+++ b/runtime/bin/file_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/file.h"
 
@@ -583,4 +583,4 @@ File::Identical File::AreIdentical(const char* file_1, const char* file_2) {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_FUCHSIA)
+#endif  // defined(HOST_OS_FUCHSIA)