[builtins] Move StringIncludes to a builtin.

test/mjsunit/es6/string-includes.js

Index: test/mjsunit/es6/string-includes.js
diff --git a/test/mjsunit/es6/string-includes.js b/test/mjsunit/es6/string-includes.js
index c825ffdc6852496f332f28b1d70b9ef01d1ace24..ae69bd668578ec25dc476b79848d27cdda8cce7d 100644
--- a/test/mjsunit/es6/string-includes.js
+++ b/test/mjsunit/es6/string-includes.js
@@ -27,6 +27,10 @@
 
 assertEquals(1, String.prototype.includes.length);
 
+var s = 'a';
+assertFalse(s.includes(null));
+assertFalse(s.includes(undefined));
+

[Franzi, 2016/10/10 12:58:58]

Can we add assertFalse(s.includes()); just to prevent mistakes in the future?

 var reString = "asdf[a-z]+(asdf)?";
 assertTrue(reString.includes("[a-z]+"));
 assertTrue(reString.includes("(asdf)?"));