Add debug mirror support for ES6 Map/Set iterators.

test/mjsunit/es6/mirror-iterators.js

+// Copyright 2014 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.
+
+// Flags: --expose-debug-as debug
+// Test the mirror object for collection iterators.
+
+function testIteratorMirror(iter, expected) {
+  var mirror = debug.MakeMirror(iter);
+  assertTrue(mirror.isIterator());
+
+  var preview = mirror.preview();
+  assertArrayEquals(expected, preview);
+
+  // Check that iterator has not changed after taking preview.
+  var values = [];
+  for (var i of iter) values.push(i);
+  assertArrayEquals(expected, values);
+}
+
+var o1 = { foo: 1 };
+var o2 = { foo: 2 };
+
+var map = new Map();
+map.set(41, 42);
+map.set(o1, o2);
+testIteratorMirror(map.keys(), [41, o1]);
+testIteratorMirror(map.values(), [42, o2]);
+testIteratorMirror(map.entries(), [[41, 42], [o1, o2]]);
+
+var set = new Set();
+set.add(41);
+set.add(42);
+set.add(o1);
+set.add(o2);
+testIteratorMirror(set.keys(), [41, 42, o1, o2]);
+testIteratorMirror(set.values(), [41, 42, o1, o2]);
+testIteratorMirror(set.entries(), [[41, 41], [42, 42], [o1, o1], [o2, o2]]);

[arv (Not doing code reviews), 2014/10/31 10:20:18]

I think it would be good to have a test that  creates a mirror after the
original iterator is halfway through.

[aandrey, 2014/10/31 10:43:22]

Done.