[es6] Implement for-of iterator finalization

test/mjsunit/harmony/generators.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 
 { // yield in try-catch
 
   let g = function*() {
     try {yield 1} catch (error) {assertEquals("caught", error)}
   };
@@ skipping 220 matching lines @@
     assertEquals({value: 43, done: false}, x.next(666));
     assertEquals({value: undefined, done: false}, x.next());
     assertEquals({value: undefined, done: true}, x.next());
   }
 
   {
     let x = g();
     assertEquals({value: 1, done: false}, x.next());
     assertEquals({value: 42, done: false}, x.next());
     assertEquals({value: 43, done: false}, x.return(666));
-    assertEquals({value: 666, done: false}, x.next());
+    assertEquals({value: undefined, done: false}, x.next());
     assertEquals({value: undefined, done: true}, x.next());
   }
 
   {
     let x = g();
     assertEquals({value: 1, done: false}, x.next());
     assertEquals({value: 42, done: false}, x.next());
     assertEquals({value: 43, done: false}, x.throw(666));
     assertThrowsEquals(() => x.next(), 666);
   }
 }