warn on missing component (#623)

src/validate/html/validateElement.ts

@@ -6,6 +6,11 @@ export default function validateElement(validator: Validator, node: Node) {
 	const isComponent =
 		node.name === ':Self' || validator.components.has(node.name);
 
+	if (!isComponent && node.name[0] === node.name[0].toUpperCase()) {
+		// TODO upgrade to validator.error in v2
+		validator.warn(`${node.name} component is not defined`, node.start);
+	}
+
 	let hasIntro: boolean;
 	let hasOutro: boolean;
 	let hasTransition: boolean;

test/validator/samples/missing-component/input.html

@@ -0,0 +1,3 @@
+<div>
+	<Widget/>
+</div>

test/validator/samples/missing-component/warnings.json

@@ -0,0 +1,8 @@
+[{
+	"message": "Widget component is not defined",
+	"loc": {
+		"line": 2,
+		"column": 1
+	},
+	"pos": 7
+}]