Ship ES2015 sloppy-mode const semantics

test/mjsunit/regress/regress-799761.js

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+// Flags: --legacy-const
+
 // const variables should be read-only
 const c = 42;
 c = 87;
 assertEquals(42, c);
 
 
 // const variables are not behaving like other JS variables when it comes
 // to scoping - in fact they behave more sanely. Inside a 'with' they do
 // not interfere with the 'with' scopes.
 
@@ skipping 45 matching lines @@
 
 
 // const variables may be declared via 'eval'
 (function () {
   with ({ x: 42 }) {
     eval('const x = 7');
   }
   x = 5;
   assertEquals(7, x);
 })();