[wasm] JSAPI conformance: instance.exports has null prototype.

BUG=v8:5885

Review-Url: https://codereview.chromium.org/2649163004
Cr-Commit-Position: refs/heads/master@{#42636}

src/factory.cc

@@ -1723,9 +1723,9 @@ Handle<JSObject> Factory::NewJSObject(Handle<JSFunction> constructor,
       isolate()->heap()->AllocateJSObject(*constructor, pretenure), JSObject);
 }
 
-
-Handle<JSObject> Factory::NewJSObjectWithNullProto() {
-  Handle<JSObject> result = NewJSObject(isolate()->object_function());
+Handle<JSObject> Factory::NewJSObjectWithNullProto(PretenureFlag pretenure) {
+  Handle<JSObject> result =
+      NewJSObject(isolate()->object_function(), pretenure);
   Handle<Map> new_map =
       Map::Copy(Handle<Map>(result->map()), "ObjectWithNullProto");
   Map::SetPrototype(new_map, null_value());

src/factory.h

@@ -475,7 +475,8 @@ class V8_EXPORT_PRIVATE Factory final {
   Handle<JSObject> NewJSObject(Handle<JSFunction> constructor,
                                PretenureFlag pretenure = NOT_TENURED);
   // JSObject without a prototype.
-  Handle<JSObject> NewJSObjectWithNullProto();
+  Handle<JSObject> NewJSObjectWithNullProto(
+      PretenureFlag pretenure = NOT_TENURED);
 
   // Global objects are pretenured and initialized based on a constructor.
   Handle<JSGlobalObject> NewJSGlobalObject(Handle<JSFunction> constructor);

src/wasm/wasm-module.cc

@@ -1912,10 +1912,7 @@ class WasmInstanceBuilder {
     Handle<JSObject> exports_object = instance;
     if (module_->origin == kWasmOrigin) {
       // Create the "exports" object.
-      Handle<JSFunction> object_function = Handle<JSFunction>(
-          isolate_->native_context()->object_function(), isolate_);
-      exports_object =
-          isolate_->factory()->NewJSObject(object_function, TENURED);
+      exports_object = isolate_->factory()->NewJSObjectWithNullProto();
       Handle<String> exports_name =
           isolate_->factory()->InternalizeUtf8String("exports");
       JSObject::AddProperty(instance, exports_name, exports_object, NONE);

test/mjsunit/wasm/js-api.js

@@ -371,6 +371,12 @@ assertTrue(instanceExportsDesc.writable);
 assertTrue(instanceExportsDesc.enumerable);
 assertTrue(instanceExportsDesc.configurable);
 
+exportsObj = exportingInstance.exports;
+assertEq(typeof exportsObj, 'object');
+assertFalse(Object.isExtensible(exportsObj));
+assertEq(Object.getPrototypeOf(exportsObj), null);
+assertEq(Object.keys(exportsObj).join(), 'f');
+
 // Exported WebAssembly functions
 let f = exportingInstance.exports.f;
 assertTrue(f instanceof Function);