For-of statements do not permit initializers.

test/mjsunit/harmony/iteration-syntax.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 36 matching lines @@
 assertThrows("function f() { for (let x of y z) { } }", SyntaxError);
 assertThrows("function f() { for (let x of y;) { } }", SyntaxError);
 
 assertThrows("function f() { for (of y) { } }", SyntaxError);
 assertThrows("function f() { for (of of) { } }", SyntaxError);
 assertThrows("function f() { for (var of y) { } }", SyntaxError);
 assertThrows("function f() { for (var of of) { } }", SyntaxError);
 assertThrows("function f() { for (let of y) { } }", SyntaxError);
 assertThrows("function f() { for (let of of) { } }", SyntaxError);
 
+assertThrows("function f() { for (x = 3 of y) { } }", SyntaxError);
+assertThrows("function f() { for (var x = 3 of y) { } }", SyntaxError);
+assertThrows("function f() { for (let x = 3 of y) { } }", SyntaxError);
+
+
 // 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) { } }