add dev mode warning when teardown is returned instead of destroy

src/generators/nodes/Element.ts

@@ -516,6 +516,14 @@ export default class Element extends Node {
 					});
 				`);
 
+				if (generator.options.dev) {
+					block.builders.hydrate.addBlock(deindent`
+						if (${handlerName}.teardown) {
+							console.warn("Return 'destroy()' from custom event handlers. Returning 'teardown()' has been deprecated and will be unsupported in Svelte 2");
+						}
+					`);
+				}
+
 				block.builders.destroy.addLine(deindent`
 					${handlerName}[${handlerName}.destroy ? 'destroy' : 'teardown']();
 				`);

test/runtime/samples/dev-warning-custom-event-destroy-not-teardown/_config.js

@@ -0,0 +1,7 @@
+export default {
+	dev: true,
+
+	warnings: [
+		`Return 'destroy()' from custom event handlers. Returning 'teardown()' has been deprecated and will be unsupported in Svelte 2`
+	]
+};

test/runtime/samples/dev-warning-custom-event-destroy-not-teardown/main.html

@@ -0,0 +1,16 @@
+<button on:foo='foo()'>foo</button>
+
+<script>
+	export default {
+		methods: {
+			foo() {}
+		},
+		events: {
+			foo(node, callback) {
+				return {
+					teardown() {}
+				}
+			}
+		}
+	};
+</script>