Split off a separate --harmony_sloppy_let flag

test/mjsunit/harmony/block-let-semantics-sloppy.js

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-// Flags: --harmony-sloppy --no-legacy-const
+// Flags: --harmony-sloppy --no-legacy-const --harmony-sloppy-let
 
 // Test temporal dead zone semantics of let bound variables in
 // function and block scopes.
 
 function TestFunctionLocal(s) {
   try {
     eval("(function(){" + s + "; })")();
   } catch (e) {
     assertInstanceof(e, ReferenceError);
     return;
@@ skipping 141 matching lines @@
     }
     eval("1 + 1");
     return x + inner();
   }
 
   let x = 1;
   return middle();
 }
 
 assertEquals(2, outer());