Base the right RegExp brand checks on Symbol.match

test/mjsunit/es6/string-startswith.js

Index: test/mjsunit/es6/string-startswith.js
diff --git a/test/mjsunit/es6/string-startswith.js b/test/mjsunit/es6/string-startswith.js
index 887db994a03fcd02ed5d79b5cf00e3e0c7ca43c3..f38f7b9cb88d991ce5eb82d77544d5fbc1c27b4d 100644
--- a/test/mjsunit/es6/string-startswith.js
+++ b/test/mjsunit/es6/string-startswith.js
@@ -399,3 +399,11 @@ assertThrows(function() {
     "toString": function() { return "abc"; }
   }, [/./]);
 }, TypeError);
+
+// startsWith does its brand checks with Symbol.match
+var re = /./;
+assertThrows(function() {
+  "".startsWith(re);
+}, TypeError);
+re[Symbol.match] = false;
+assertEquals(false, "".startsWith(re));