ES6: Add missing Set.prototype.keys function

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-set.prototype.keys

The value of the keys property is just the initial value of the values
function.

BUG=v8:3411
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/353293003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/collection-iterator.js

@@ -87,6 +87,7 @@ function ExtendSetPrototype() {
 
   InstallFunctions($Set.prototype, DONT_ENUM, $Array(
     'entries', SetEntries,
+    'keys', SetValues,
     'values', SetValues
   ));
 

test/mjsunit/harmony/collection-iterator.js

@@ -37,6 +37,11 @@
 })();
 
 
+(function TestSetIteratorKeys() {
+  assertEquals(Set.prototype.keys, Set.prototype.values);
+})();
+
+
 (function TestSetIteratorEntries() {
   var s = new Set;
   s.add(1);