Retain maps that have live prototypes.

test/cctest/test-heap.cc

 // 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 5083 matching lines @@
 
 
 void CheckMapRetainingFor(int n) {
   FLAG_retain_maps_for_n_gc = n;
   Isolate* isolate = CcTest::i_isolate();
   Heap* heap = isolate->heap();
   Handle<WeakCell> weak_cell;
   {
     HandleScope inner_scope(isolate);
     Handle<Map> map = Map::Create(isolate, 1);
+    v8::Local<v8::Value> result =
+        CompileRun("(function () { return {x : 10}; })();");
+    Handle<JSObject> proto =
+        v8::Utils::OpenHandle(*v8::Handle<v8::Object>::Cast(result));
+    map->set_prototype(*proto);
     heap->AddRetainedMap(map);
     weak_cell = inner_scope.CloseAndEscape(Map::WeakCellForMap(map));
   }
   CHECK(!weak_cell->cleared());
   for (int i = 0; i < n; i++) {
     heap->CollectGarbage(OLD_POINTER_SPACE);
   }
   CHECK(!weak_cell->cleared());
   heap->CollectGarbage(OLD_POINTER_SPACE);
   CHECK(weak_cell->cleared());
@@ skipping 24 matching lines @@
 
 TEST(WritableVsImmortalRoots) {
   for (int i = 0; i < Heap::kStrongRootListLength; ++i) {
     Heap::RootListIndex root_index = static_cast<Heap::RootListIndex>(i);
     bool writable = Heap::RootCanBeWrittenAfterInitialization(root_index);
     bool immortal = Heap::RootIsImmortalImmovable(root_index);
     // A root value can be writable, immortal, or neither, but not both.
     CHECK(!immortal || !writable);
   }
 }