augment warnings to help people figure out they need to add a <script>

src/compile/Component.ts

@@ -1143,9 +1143,12 @@ export default class Component {
 		if (template_scope && template_scope.names.has(name)) return;
 		if (globalWhitelist.has(name)) return;
 
+		let message = `'${name}' is not defined`;
+		if (!this.ast.instance) message += `. Consider adding a <script> block with 'export let ${name}' to declare a prop`;
+
 		this.warn(node, {
 			code: 'missing-declaration',
-			message: `'${name}' is not defined`
+			message
 		});
 	}
 }

test/validator/samples/action-invalid/warnings.json

@@ -1,6 +1,6 @@
 [{
 	"code": "missing-declaration",
-	"message": "'whatever' is not defined",
+	"message": "'whatever' is not defined. Consider adding a <script> block with 'export let whatever' to declare a prop",
 	"pos": 5,
 	"start": {
 		"line": 1,

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

@@ -1,6 +1,6 @@
 [{
 	"code": "missing-declaration",
-	"message": "'Widget' is not defined",
+	"message": "'Widget' is not defined. Consider adding a <script> block with 'export let Widget' to declare a prop",
 	"start": {
 		"line": 2,
 		"column": 1,

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

@@ -1,6 +1,6 @@
 [{
 	"code": "missing-declaration",
-	"message": "'foo' is not defined",
+	"message": "'foo' is not defined. Consider adding a <script> block with 'export let foo' to declare a prop",
 	"start": {
 		"line": 1,
 		"column": 5,