Implement Instance instances correctly; fix a few error cases

test/mjsunit/wasm/instantiate-module-basic.js

 // Copyright 2015 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
 
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
 let kReturnValue = 17;
@@ skipping 204 matching lines @@
     ]).exportFunc();
 
   var mem_1 = new WebAssembly.Memory({initial: 1});
   var mem_2 = new WebAssembly.Memory({initial: 1});
   var view_1 = new Int32Array(mem_1.buffer);
   var view_2 = new Int32Array(mem_2.buffer);
   view_1[0] = 1;
   view_2[0] = 1000;
 
   var module = new WebAssembly.Module(builder.toBuffer());
-  var i1 = new WebAssembly.Instance(module, null, mem_1);
-  var i2 = new WebAssembly.Instance(module, null, mem_2);
+  var i1 = new WebAssembly.Instance(module, undefined, mem_1);
+  var i2 = new WebAssembly.Instance(module, undefined, mem_2);
 
   assertEquals(1, i1.exports.f());
   assertEquals(1000, i2.exports.f());
 })();
 
 (function MustBeMemory() {
   print("MustBeMemory...");
   var memory = new ArrayBuffer(65536);
   var module = new WebAssembly.Module(buffer);
-  assertThrows(() => new WebAssembly.Instance(module, null, memory), TypeError);
+  assertThrows(
+      () => new WebAssembly.Instance(module, undefined, memory), TypeError);
 })();