[regexp] Fix regexp source escaping with preceding backslashes.

test/mjsunit/regress/regress-crbug-515897.js

Index: test/mjsunit/regress/regress-crbug-515897.js
diff --git a/test/intl/regexp-prepare.js b/test/mjsunit/regress/regress-crbug-515897.js
similarity index 50%
copy from test/intl/regexp-prepare.js
copy to test/mjsunit/regress/regress-crbug-515897.js
index dec84110ed02e30bba674538534a069f7d43d39d..45a812c781ab13e37529e538c0ccac6fa65f11ff 100644
--- a/test/intl/regexp-prepare.js
+++ b/test/mjsunit/regress/regress-crbug-515897.js
@@ -2,4 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/(\w)(\w)(\w)(\w)(\w)(\w)(\w)(\w)(\w)(\w)/.exec(">>>abcdefghij<<<");
+var r1 = new RegExp("\\\\/");
+assertTrue(r1.test("\\/"));
+var r2 = eval("/" + r1.source + "/");
+assertEquals("\\\\\\/", r1.source);
+assertEquals("\\\\\\/", r2.source);