Index: libcxx/utils/libcxx/test/config.py
===================================================================
--- libcxx/utils/libcxx/test/config.py
+++ libcxx/utils/libcxx/test/config.py
@@ -204,7 +204,7 @@
 
         assert self.cxx_is_clang_cl
         flags = []
-        compile_flags = _prefixed_env_list('INCLUDE', '-isystem')
+        compile_flags = []
         link_flags = _prefixed_env_list('LIB', '-L')
         for path in _split_env_var('LIB'):
             self.add_path(self.exec_env, path)