Implement Instance instances correctly; fix a few error cases

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 69118b31c42913b7fbd6a4cd34e705abba3c1559..01aa389d2daa9acade73ed076b143d0d6fabf7c4 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -4148,8 +4148,6 @@ bool Genesis::InstallSpecialObjects(Handle<Context> native_context) {
 
   Factory* factory = isolate->factory();
   HandleScope scope(isolate);
-  Handle<JSGlobalObject> global(JSGlobalObject::cast(
-      native_context->global_object()));
 
   Handle<JSObject> Error = isolate->error_function();
   Handle<String> name =
@@ -4157,7 +4155,9 @@ bool Genesis::InstallSpecialObjects(Handle<Context> native_context) {
   Handle<Smi> stack_trace_limit(Smi::FromInt(FLAG_stack_trace_limit), isolate);
   JSObject::AddProperty(Error, name, stack_trace_limit, NONE);
 
-  WasmJs::Install(isolate, global);
+  if (FLAG_expose_wasm || FLAG_validate_asm) {
+    WasmJs::Install(isolate);
+  }
 
   return true;
 }