skip observedAttributes where appropriate - fixes #2399

Author: Rich-Harris

src/compile/render-dom/index.ts

@@ -464,9 +464,10 @@ export default function dom(
 					}
 				}
 
+				${props.length > 0 && deindent`
 				static get observedAttributes() {
 					return ${JSON.stringify(props.map(x => x.export_name))};
-				}
+				}`}
 
 				${body.length > 0 && body.join('\n\n')}
 			}

test/js/samples/css-shadow-dom-keyframes/expected.js

@@ -49,10 +49,6 @@ class Component extends SvelteElement {
 			}
 		}
 	}
-
-	static get observedAttributes() {
-		return [];
-	}
 }
 
 customElements.define("custom-element", Component);