Use spread syntax (convert to es2015) (#6317)

src/document/doc-debug.js

@@ -7,7 +7,7 @@ function flattenDoc(doc) {
     for (let i = 0; i < doc.parts.length; ++i) {
       const doc2 = doc.parts[i];
       if (typeof doc2 !== "string" && doc2.type === "concat") {
-        [].push.apply(res, flattenDoc(doc2).parts);
+        res.push(...flattenDoc(doc2).parts);
       } else {
         const flattened = flattenDoc(doc2);
         if (flattened !== "") {