Ship ES2015 sloppy-mode const semantics

test/mjsunit/compiler/regress-const.js

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-// Flags: --allow-natives-syntax
+// Flags: --allow-natives-syntax --legacy-const
 
 // Test const initialization and assignments.
 function f() {
   var x = 42;
   while (true) {
     const y = x;
     if (--x == 0) return y;
   }
 }
 
@@ skipping 20 matching lines @@
   const X = 42;
   return r + X;
 }
 
 for (var i = 0; i < 5; i++) h(1,2);
 
 %OptimizeFunctionOnNextCall(h);
 
 assertEquals(45, h(1,2));
 assertEquals("foo742", h("foo", 7));