Implement Instance instances correctly; fix a few error cases

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

Index: test/mjsunit/wasm/instantiate-module-basic.js
diff --git a/test/mjsunit/wasm/instantiate-module-basic.js b/test/mjsunit/wasm/instantiate-module-basic.js
index 927851e8682184dac383070d4aa7a6bb2cf2ff55..38302cd15a469abd71a269d4b8f694f7dddbb787 100644
--- a/test/mjsunit/wasm/instantiate-module-basic.js
+++ b/test/mjsunit/wasm/instantiate-module-basic.js
@@ -222,8 +222,8 @@ assertFalse(WebAssembly.validate(bytes(88, 88, 88, 88, 88, 88, 88, 88)));
   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());
@@ -233,5 +233,6 @@ assertFalse(WebAssembly.validate(bytes(88, 88, 88, 88, 88, 88, 88, 88)));
   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);
 })();