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 !== "") {

src/document/doc-printer.js

@@ -491,7 +491,7 @@ function printDocToString(doc, options) {
             case MODE_BREAK:
               if (lineSuffix.length) {
                 cmds.push([ind, mode, doc]);
-                [].push.apply(cmds, lineSuffix.reverse());
+                cmds.push(...lineSuffix.reverse());
                 lineSuffix = [];
                 break;
               }

src/language-html/utils.js

@@ -578,7 +578,7 @@ function normalizeParts(parts) {
     }
 
     if (part.type === "concat") {
-      Array.prototype.unshift.apply(restParts, part.parts);
+      restParts.unshift(...part.parts);
       continue;
     }