[regexp] Avoid side effects between map load and fast path check

test/mjsunit/regress/regress-709029.js

Index: test/mjsunit/regress/regress-709029.js
diff --git a/test/mjsunit/regress/regress-709029.js b/test/mjsunit/regress/regress-709029.js
new file mode 100644
index 0000000000000000000000000000000000000000..b9bcabbc27b77b5c7f944eb50999c4a7613f4d52
--- /dev/null
+++ b/test/mjsunit/regress/regress-709029.js
@@ -0,0 +1,38 @@
+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Changes the map of obj.
+function mutateObjectOnStringConversion(obj) {
+  return { toString: () => { obj.x = 42;return "";}};
+}
+
+{
+  const re = /./;
+  re.exec(mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re.test(mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.match](mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.search](mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.split](mutateObjectOnStringConversion(re));
+}
+
+{
+  const re = /./;
+  re[Symbol.replace](mutateObjectOnStringConversion(re));
+}