Add optional max elements limit for Map/Set mirror iterator preview.

test/mjsunit/es6/mirror-collections.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 --expose-gc
 
 function testMapMirror(mirror) {
   // Create JSON representation.
   var serializer = debug.MakeMirrorSerializer();
   var json = JSON.stringify(serializer.serializeValue(mirror));
@@ skipping 33 matching lines @@
 var o2 = new Object();
 var o3 = new Object();
 
 // Test the mirror object for Maps
 var map = new Map();
 map.set(o1, 11);
 map.set(o2, 22);
 map.delete(o1);
 var mapMirror = debug.MakeMirror(map);
 testMapMirror(mapMirror);
+
 var entries = mapMirror.entries();
 assertEquals(1, entries.length);
 assertSame(o2, entries[0].key);
 assertEquals(22, entries[0].value);
 map.set(o1, 33);
 map.set(o3, o2);
 map.delete(o2);
 map.set(undefined, 44);
+
 entries = mapMirror.entries();
 assertEquals(3, entries.length);
 assertSame(o1, entries[0].key);
 assertEquals(33, entries[0].value);
 assertSame(o3, entries[1].key);
 assertSame(o2, entries[1].value);
 assertEquals(undefined, entries[2].key);
 assertEquals(44, entries[2].value);
 
+assertEquals(3, mapMirror.entries(0).length);
+assertEquals(1, mapMirror.entries(1).length);
+assertEquals(2, mapMirror.entries(2).length);
+
 // Test the mirror object for Sets
 var set = new Set();
 set.add(o1);
 set.add(o2);
 set.delete(o1);
 set.add(undefined);
 var setMirror = debug.MakeMirror(set);
 testSetMirror(setMirror);
 var values = setMirror.values();
 assertEquals(2, values.length);
+assertEquals(1, setMirror.values(1).length);
 assertSame(o2, values[0]);
 assertEquals(undefined, values[1]);
 
 // Test the mirror object for WeakMaps
 var weakMap = new WeakMap();
 weakMap.set(o1, 11);
 weakMap.set(new Object(), 22);
 weakMap.set(o3, 33);
 weakMap.set(new Object(), 44);
 var weakMapMirror = debug.MakeMirror(weakMap);
 testMapMirror(weakMapMirror);
 weakMap.set(new Object(), 55);
 assertTrue(weakMapMirror.entries().length <= 5);
 gc();
 
 function testWeakMapEntries(weakMapMirror) {
   var entries = weakMapMirror.entries();
   assertEquals(2, entries.length);
+  assertEquals(2, weakMapMirror.entries(0).length);
+  assertEquals(1, weakMapMirror.entries(1).length);
   var found = 0;
   for (var i = 0; i < entries.length; i++) {
     if (Object.is(entries[i].key, o1)) {
       assertEquals(11, entries[i].value);
       found++;
     }
     if (Object.is(entries[i].key, o3)) {
       assertEquals(33, entries[i].value);
       found++;
     }
@@ skipping 13 matching lines @@
 weakSet.add(o3);
 weakSet.delete(o2);
 var weakSetMirror = debug.MakeMirror(weakSet);
 testSetMirror(weakSetMirror);
 assertTrue(weakSetMirror.values().length <= 5);
 gc();
 
 function testWeakSetValues(weakSetMirror) {
   var values = weakSetMirror.values();
   assertEquals(2, values.length);
+  assertEquals(2, weakSetMirror.values(0).length);
+  assertEquals(1, weakSetMirror.values(1).length);
   var found = 0;
   for (var i = 0; i < values.length; i++) {
     if (Object.is(values[i], o1)) {
       found++;
     }
     if (Object.is(values[i], o3)) {
       found++;
     }
   }
   assertEquals(2, found);
 }
 
 testWeakSetValues(weakSetMirror);