add dev mode missing prop check

src/compile/render-dom/index.ts

@@ -102,7 +102,7 @@ export default function dom(
 
 	const body = [];
 
-	const debug_name = `<${component.customElement ? component.tag : name}>`;
+	const debug_name = `<${component.customElement ? component.customElement.tag : name}>`;
 	const not_equal = component.options.immutable ? `@not_equal` : `@safe_not_equal`;
 	let dev_props_check;
 
@@ -141,7 +141,7 @@ export default function dom(
 				const state = this.$$.get();
 				${expected.map(name => deindent`
 
-				if (state.${name} === undefined) {
+				if (state.${name} === undefined${component.customElement && ` && !('${name}' in this.attributes)`}) {
 					console.warn("${debug_name} was created without expected data property '${name}'");
 				}`)}
 			`;
@@ -184,6 +184,8 @@ export default function dom(
 
 					@init(this, { target: this.shadowRoot }, define, create_fragment, ${not_equal});
 
+					${dev_props_check}
+
 					if (options) {
 						if (options.target) {
 							@insert(options.target, this, options.anchor);

test/custom-elements/samples/nested/Counter.html

@@ -1,7 +0,0 @@
-<svelte:meta tag="my-counter"/>
-
-<script>
-	export let count = 0;
-</script>
-
-<button on:click='{() => count += 1}'>count: {count}</button>

test/custom-elements/samples/nested/main.html

@@ -1,10 +0,0 @@
-<svelte:meta tag="my-app"/>
-
-<script>
-	import Counter from './Counter.html';
-
-	export let count;
-</script>
-
-<Counter bind:count/>
-<p>clicked {count} times</p>

test/custom-elements/samples/nested/test.js

@@ -1,17 +0,0 @@
-import * as assert from 'assert';
-import './main.html';
-
-export default async function (target) {
-	target.innerHTML = '<my-app/>';
-	const el = target.querySelector('my-app');
-	const counter = el.shadowRoot.querySelector('my-counter');
-	const button = counter.shadowRoot.querySelector('button');
-
-	assert.equal(counter.count, 0);
-	assert.equal(counter.shadowRoot.innerHTML, `<button>count: 0</button>`);
-
-	await button.dispatchEvent(new MouseEvent('click'));
-
-	assert.equal(counter.count, 1);
-	assert.equal(counter.shadowRoot.innerHTML, `<button>count: 1</button>`);
-}