Add missed changes for Symbols as WeakMap keys proposal.

Co-authored-by: Rick Waldron <waldron.rick@gmail.com>

test/built-ins/WeakMap/iterator-items-are-not-object-close-iterator.js

@@ -68,7 +68,7 @@ assert.throws(TypeError, function() {
 });
 assert.sameValue(count, 5);
 
-nextItem = Symbol('a');
+nextItem = Symbol.for('a');
 assert.throws(TypeError, function() {
   new WeakMap(iterable);
 });

test/built-ins/WeakSet/prototype/add/returns-this-symbol.js

@@ -0,0 +1,18 @@
+// Copyright (C) 2022 Igalia S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-weakset.prototype.add
+description: Returns `this` after adding a new value.
+info: |
+  WeakSet.prototype.add ( value )
+
+  1. Let S be the this value.
+  ...
+  7. Return S.
+features: [Symbol, WeakSet, symbols-as-weakmap-keys]
+---*/
+
+var s = new WeakSet();
+
+assert.sameValue(s.add(Symbol('description')), s, '`s.add(Symbol("description"))` returns `s`');

test/built-ins/WeakSet/prototype/add/returns-this-when-ignoring-duplicate-symbol.js

@@ -0,0 +1,22 @@
+// Copyright (C) 2022 Igalia S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-weakset.prototype.add
+description: Returns `this` when new value is duplicate.
+info: |
+  WeakSet.prototype.add ( value )
+
+  1. Let S be the this value.
+  ...
+  5. 5. For each element e of entries, do
+    a. If e is not empty and SameValue(e, value) is true, then
+      i. Return S.
+  ...
+features: [Symbol, WeakSet, symbols-as-weakmap-keys]
+---*/
+
+var foo = Symbol('description');
+var s = new WeakSet([foo]);
+
+assert.sameValue(s.add(foo), s, '`s.add(foo)` returns `s`');