Make Map.set() and Set.add() chainable

test/mjsunit/es6/weak_collections.js

 // Copyright 2012 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 50 matching lines @@
   assertThrows(function () { m.set(0, 0) }, TypeError);
   assertThrows(function () { m.get('a-key') }, TypeError);
   assertThrows(function () { m.set('a-key', 0) }, TypeError);
 }
 TestInvalidCalls(new WeakMap);
 
 
 // Test expected behavior for WeakSets
 function TestSet(set, key) {
   assertFalse(set.has(key));
-  assertSame(undefined, set.add(key));
+  assertSame(set, set.add(key));
   assertTrue(set.has(key));
   assertTrue(set.delete(key));
   assertFalse(set.has(key));
   assertFalse(set.delete(key));
   assertFalse(set.has(key));
 }
 function TestSetBehavior(set) {
   for (var i = 0; i < 20; i++) {
     TestSet(set, new Object);
     TestSet(set, i);
     TestSet(set, i / 100);
     TestSet(set, 'key-' + i);
   }
   var keys = [ +0, -0, +Infinity, -Infinity, true, false, null, undefined ];
   for (var i = 0; i < keys.length; i++) {
     TestSet(set, keys[i]);
   }
 }
 TestSet(new WeakSet, new Object);
 
 
 // Test expected mapping behavior for WeakMaps
 function TestMapping(map, key, value) {
-  assertSame(undefined, map.set(key, value));
+  assertSame(map, map.set(key, value));
   assertSame(value, map.get(key));
 }
 function TestMapBehavior1(m) {
   TestMapping(m, new Object, 23);
   TestMapping(m, new Object, 'the-value');
   TestMapping(m, new Object, new Object);
 }
 TestMapBehavior1(new WeakMap);
 
 
@@ skipping 218 matching lines @@
 
 // Test WeakSet clear
 (function() {
   var k = new Object();
   var w = new WeakSet();
   w.add(k);
   assertTrue(w.has(k));
   w.clear();
   assertFalse(w.has(k));
 })();