Fix concat bug

src/lib/blocks.js

@@ -45,7 +45,7 @@ export default function (vm) {
     const backdropsMenu = function () {
         if (vm.runtime.targets[0] && vm.runtime.targets[0].sprite.costumes.length > 0) {
             return vm.runtime.targets[0].sprite.costumes.map(costume => [costume.name, costume.name])
-                .concat(['next backdrop', '_nextBackdrop_'], ['previous backdrop', '_prevBackdrop_']);
+                .concat([['next backdrop', '_nextBackdrop_'], ['previous backdrop', '_prevBackdrop_']]);
         }
         return [['', '']];
     };