perf: partially replace .concat with .push (#13609)

packages/babel-plugin-proposal-async-generator-functions/src/index.js

@@ -47,7 +47,7 @@ export default declare(api => {
       }
 
       // push the rest of the original loop body onto our new body
-      block.body = block.body.concat(node.body.body);
+      block.body.push(...node.body.body);
 
       t.inherits(loop, node);
       t.inherits(loop.body, node.body);