[string] Move String.p.toLowerCase to CSA

test/mjsunit/string-case.js

Index: test/mjsunit/string-case.js
diff --git a/test/mjsunit/string-case.js b/test/mjsunit/string-case.js
index 34c2340d33c2ff676549e8a571a149dc9b4b26dc..b6934eb705ed8bd4702d4bafabe02507aaa0c729 100644
--- a/test/mjsunit/string-case.js
+++ b/test/mjsunit/string-case.js
@@ -59,8 +59,9 @@ function test(length) {
     strLower += String.fromCharCode(charCodeToLower(c));
     strUpper += String.fromCharCode(charCodeToUpper(c));
   }
-  %FlattenString(strLower);
-  %FlattenString(strUpper);
+  str = %FlattenString(str);
+  strLower = %FlattenString(strLower);
+  strUpper = %FlattenString(strUpper);
   // Sequential string.
   assertEquals(strLower, str.toLowerCase());
   assertEquals(strUpper, str.toUpperCase());