Implement Instance instances correctly; fix a few error cases

test/mjsunit/wasm/module-memory.js

Index: test/mjsunit/wasm/module-memory.js
diff --git a/test/mjsunit/wasm/module-memory.js b/test/mjsunit/wasm/module-memory.js
index d7cc1297c92bca84d7985d6b28d2aeca5d94e71e..9f87a31d3288c8e16f5f30f3241b77d57053999a 100644
--- a/test/mjsunit/wasm/module-memory.js
+++ b/test/mjsunit/wasm/module-memory.js
@@ -36,7 +36,7 @@ function genModule(memory) {
       /**/ kExprI32Const, 0                // --
     ])
     .exportFunc();
-  var module = builder.instantiate(null, memory);
+  var module = builder.instantiate(undefined, memory);
   assertTrue(module.exports.memory instanceof WebAssembly.Memory);
   if (memory != null) assertEquals(memory.buffer, module.exports.memory.buffer);
   return module;
@@ -44,7 +44,7 @@ function genModule(memory) {
 
 function testPokeMemory() {
   print("testPokeMemory");
-  var module = genModule(null);
+  var module = genModule(undefined);
   var buffer = module.exports.memory.buffer;
   var main = module.exports.main;
   assertEquals(kMemSize, buffer.byteLength);
@@ -75,7 +75,7 @@ function genAndGetMain(buffer) {
 }
 
 function testSurvivalAcrossGc() {
-  var checker = genAndGetMain(null);
+  var checker = genAndGetMain(undefined);
   for (var i = 0; i < 3; i++) {
     print("gc run ", i);
     assertEquals(0, checker(kMemSize - 4));