Make `let` usable as an identifier in ES6 sloppy mode.

test/mjsunit/harmony/iteration-syntax.js

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-// Flags: --harmony-iteration --harmony-scoping
+// Flags: --harmony-iteration --harmony-scoping --use-strict
 
 // Test for-of syntax.
 
 "use strict";
 
 function f() { for (x of y) { } }
 function f() { for (var x of y) { } }
 function f() { for (let x of y) { } }
 
 assertThrows("function f() { for (x of) { } }", SyntaxError);
@@ skipping 22 matching lines @@
 
 // Alack, this appears to be valid.
 function f() { for (of of y) { } }
 function f() { for (let of of y) { } }
 function f() { for (var of of y) { } }
 
 // This too, of course.
 function f() { for (of in y) { } }
 function f() { for (var of in y) { } }
 function f() { for (let of in y) { } }