let this point to instance

src/runtime/internal/Component.ts

@@ -127,7 +127,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 	let ready = false;
 
 	$$.ctx = instance
-		? instance(component, prop_values, (i, ret, ...rest) => {
+		? instance.call(component, component, prop_values, (i, ret, ...rest) => {
 			const value = rest.length ? rest[0] : ret;
 			if ($$.ctx && not_equal($$.ctx[i], $$.ctx[i] = value)) {
 				if ($$.bound[i]) $$.bound[i](value);

test/custom-elements/samples/instance-this/main.svelte

@@ -0,0 +1,7 @@
+<svelte:options tag="my-app"/>
+
+<script>
+	export let self;
+
+	self = this;
+</script>

test/custom-elements/samples/instance-this/test.js

@@ -0,0 +1,8 @@
+import * as assert from 'assert';
+import './main.svelte';
+
+export default function (target) {
+	target.innerHTML = '<my-app />';
+	const el = target.querySelector('my-app');
+	assert.ok(el.self === el);
+}