[strong] dot prototypes of strong class literals should be strong objects

BUG=v8:3956
LOG=N

Review URL: https://codereview.chromium.org/1270423003

Cr-Commit-Position: refs/heads/master@{#30032}

src/runtime/runtime-classes.cc

@@ -137,6 +137,9 @@ static MaybeHandle<Object> DefineClass(Isolate* isolate, Handle<Object> name,
 
   Handle<Map> map =
       isolate->factory()->NewMap(JS_OBJECT_TYPE, JSObject::kHeaderSize);
+  if (constructor->map()->is_strong()) {
+    map->set_is_strong();
+  }
   Map::SetPrototype(map, prototype_parent);
   map->SetConstructor(*constructor);
   Handle<JSObject> prototype = isolate->factory()->NewJSObjectFromMap(map);

test/mjsunit/strong/literals.js

@@ -310,8 +310,8 @@ let GeneratorPrototype = (function*(){}).__proto__;
   'use strong';
   function assertStrongClass(x) {
     assertTrue(%IsStrong(x));
-    // TODO(rossberg): strongify class prototype and instance
-    // assertTrue(%IsStrong(x.prototype));
+    assertTrue(%IsStrong(x.prototype));
+    // TODO(rossberg): strongify class instance
     // assertTrue(%IsStrong(new x));
   }
   class C {};