add dev mode warning for double destroy

src/generators/dom/index.ts

@@ -264,7 +264,7 @@ export default function dom(
 		};
 
 		${name}.prototype.teardown = ${name}.prototype.destroy = function destroy ( detach ) {
-			if ( this._destroyed ) return;
+			if ( this._destroyed ) return${options.dev && ` console.warn( 'Component was already destroyed' )`};
 			this.fire( 'destroy' );
 			${templateProperties.ondestroy && `@template.ondestroy.call( this );`}
 

test/runtime/samples/dev-warning-destroy-twice/_config.js

@@ -0,0 +1,21 @@
+export default {
+	dev: true,
+
+	test(assert, component) {
+		const warn = console.warn; // eslint-disable-line no-console
+
+		const warnings = [];
+		console.warn = warning => { // eslint-disable-line no-console
+			warnings.push(warning);
+		};
+
+		component.destroy();
+		component.destroy();
+
+		assert.deepEqual(warnings, [
+			`Component was already destroyed`
+		]);
+
+		console.warn = warn; // eslint-disable-line no-console
+	}
+};

test/runtime/samples/dev-warning-destroy-twice/main.html

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