Rename TARGET_OS_* to HOST_OS_*.

runtime/bin/eventhandler.h

Index: runtime/bin/eventhandler.h
diff --git a/runtime/bin/eventhandler.h b/runtime/bin/eventhandler.h
index 50852cd4d1e1b7c32943a83771a9a2cb30a26abd..f410ee9ed0182c506df18e59f94a0ef8cbda953d 100644
--- a/runtime/bin/eventhandler.h
+++ b/runtime/bin/eventhandler.h
@@ -605,15 +605,15 @@ class DescriptorInfoMultipleMixin : public DI {
 }  // namespace dart
 
 // The event handler delegation class is OS specific.
-#if defined(TARGET_OS_ANDROID)
+#if defined(HOST_OS_ANDROID)
 #include "bin/eventhandler_android.h"
-#elif defined(TARGET_OS_FUCHSIA)
+#elif defined(HOST_OS_FUCHSIA)
 #include "bin/eventhandler_fuchsia.h"
-#elif defined(TARGET_OS_LINUX)
+#elif defined(HOST_OS_LINUX)
 #include "bin/eventhandler_linux.h"
-#elif defined(TARGET_OS_MACOS)
+#elif defined(HOST_OS_MACOS)
 #include "bin/eventhandler_macos.h"
-#elif defined(TARGET_OS_WINDOWS)
+#elif defined(HOST_OS_WINDOWS)
 #include "bin/eventhandler_win.h"
 #else
 #error Unknown target os.