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

test/mjsunit/wasm/js-api.js

 // Copyright 2016 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-wasm --allow-natives-syntax
 
 if ((typeof drainJobQueue) != "function") {
   drainJobQueue = () => { %RunMicrotasks() };
 }
 
@@ skipping 353 matching lines @@
 assertEq(String(exportingInstance), "[object WebAssembly.Instance]");
 assertEq(Object.getPrototypeOf(exportingInstance), instanceProto);
 
 // 'WebAssembly.Instance' 'exports' data property
 let instanceExportsDesc = Object.getOwnPropertyDescriptor(exportingInstance, 'exports');
 assertEq(typeof instanceExportsDesc.value, "object");
 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);
 assertEq(f.length, 0);
 assertTrue('name' in f);
 assertEq(Function.prototype.call.call(f), 42);
 assertErrorMessage(() => new f(), TypeError, /is not a constructor/);
 
 // 'WebAssembly.Memory' data property
 let memoryDesc = Object.getOwnPropertyDescriptor(WebAssembly, 'Memory');
@@ skipping 316 matching lines @@
     assertTrue(result.instance instanceof Instance);
   }
 }
 assertInstantiateSuccess(emptyModule);
 assertInstantiateSuccess(emptyModuleBinary);
 assertInstantiateSuccess(emptyModuleBinary.buffer);
 assertInstantiateSuccess(importingModule, {'': {f: () => {}}});
 assertInstantiateSuccess(importingModuleBinary, {"":{f:()=>{}}});
 assertInstantiateSuccess(importingModuleBinary.buffer, {"":{f:()=>{}}});
 assertInstantiateSuccess(memoryImportingModuleBinary, {"": {"my_memory": scratch_memory}});