Make sure that names of temporaries do not clash with real variables.

test/mjsunit/regress/regress-2322.js

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 // Flags: --harmony-scoping
 
+"use strict";
+
 assertThrows("'use strict'; for (let x in x);", ReferenceError);
 
+let s;
+for (let pppp in {}) {};
+assertThrows(function() { pppp = true }, ReferenceError);