[builtins] Move StringStartsWith to a C++ builtin.

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 f38f7b9cb88d991ce5eb82d77544d5fbc1c27b4d..449d07a56d18cbe67428bdef3fdf818c82a5e256 100644
--- a/test/mjsunit/es6/string-startswith.js
+++ b/test/mjsunit/es6/string-startswith.js
@@ -34,6 +34,7 @@ assertFalse(testString.startsWith("hello"));
 assertFalse(testString.startsWith("Hello World!"));
 assertFalse(testString.startsWith(null));
 assertFalse(testString.startsWith(undefined));
+assertFalse(testString.startsWith());
 
 assertTrue("null".startsWith(null));
 assertTrue("undefined".startsWith(undefined));