Update HOST_OS guard names (#8)

root_certificates.cc

diff --git a/root_certificates.cc b/root_certificates.cc
index afebe71..4e8c5e2 100644
--- a/root_certificates.cc
+++ b/root_certificates.cc
@@ -9,7 +9,7 @@
 #if !defined(DART_IO_DISABLED) && !defined(DART_IO_SECURE_SOCKET_DISABLED)
 
 #include "platform/globals.h"
-#if !defined(HOST_OS_MACOS) && !defined(HOST_OS_ANDROID)
+#if !defined(DART_HOST_OS_MACOS) && !defined(DART_HOST_OS_ANDROID)
 
 namespace dart {
 namespace bin {
@@ -17863,7 +17863,7 @@
 }  // namespace bin
 }  // namespace dart
 
-#endif  // !defined(HOST_OS_MACOS) && !defined(HOST_OS_ANDROID)
+#endif  // !defined(DART_HOST_OS_MACOS) && !defined(DART_HOST_OS_ANDROID)
 
 #endif  // !defined(DART_IO_DISABLED) &&
         // !defined(DART_IO_SECURE_SOCKET_DISABLED)