use IE-compatible js to check for unknown prop warnings (#3914)

src/compiler/compile/render_dom/index.ts

@@ -247,7 +247,7 @@ export default function dom(
 			function create_fragment(#ctx) {
 				${block.get_contents()}
 			}
-		`);	
+		`);
 	}
 
 	body.push(b`
@@ -369,7 +369,7 @@ export default function dom(
 			unknown_props_check = b`
 				const writable_props = [${writable_props.map(prop => x`'${prop.export_name}'`)}];
 				@_Object.keys($$props).forEach(key => {
-					if (!writable_props.includes(key) && !key.startsWith('$$')) @_console.warn(\`<${component.tag}> was created with unknown prop '\${key}'\`);
+					if (!~writable_props.indexOf(key) && key.slice(0, 2) !== '$$') @_console.warn(\`<${component.tag}> was created with unknown prop '\${key}'\`);
 				});
 			`;
 		}