Ship ES2015 sloppy-mode const semantics

test/mjsunit/const.js

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+// Flags: --legacy-const
+
 // Test const properties and pre/postfix operation.
 function f() {
   const x = 1;
   x++;
   assertEquals(1, x);
   x--;
   assertEquals(1, x);
   ++x;
   assertEquals(1, x);
   --x;
@@ skipping 23 matching lines @@
   assertEquals(5, valueOfCount);
   o--;
   assertEquals(42, +o);
   assertEquals(7, valueOfCount);
   --o;
   assertEquals(42, +o);
   assertEquals(9, valueOfCount);
 }
 
 g();