Ensure correct key order (#6616)

* Ensure correct key order

* Fix bug

* Format this document while we're at it

packages/next/build/webpack/plugins/build-manifest-plugin.js

 import { RawSource } from 'webpack-sources'
-import { BUILD_MANIFEST, ROUTE_NAME_REGEX, IS_BUNDLED_PAGE_REGEX, CLIENT_STATIC_FILES_RUNTIME_MAIN } from 'next-server/constants'
+import {
+  BUILD_MANIFEST,
+  ROUTE_NAME_REGEX,
+  IS_BUNDLED_PAGE_REGEX,
+  CLIENT_STATIC_FILES_RUNTIME_MAIN
+} from 'next-server/constants'
 
 // This plugin creates a build-manifest.json for all assets that are being output
 // It has a mapping of "entry" filename to real filename. Because the real filename can be hashed in production
 export default class BuildManifestPlugin {
   apply (compiler) {
-    compiler.hooks.emit.tapAsync('NextJsBuildManifest', (compilation, callback) => {
+    compiler.hooks.emit.tapAsync(
+      'NextJsBuildManifest',
+      (compilation, callback) => {
         const { chunks } = compilation
         const assetMap = { devFiles: [], pages: {} }
 
-      const mainJsChunk = chunks.find((c) => c.name === CLIENT_STATIC_FILES_RUNTIME_MAIN)
-      const mainJsFiles = mainJsChunk && mainJsChunk.files.length > 0 ? mainJsChunk.files.filter((file) => /\.js$/.test(file)) : []
+        const mainJsChunk = chunks.find(
+          c => c.name === CLIENT_STATIC_FILES_RUNTIME_MAIN
+        )
+        const mainJsFiles =
+          mainJsChunk && mainJsChunk.files.length > 0
+            ? mainJsChunk.files.filter(file => /\.js$/.test(file))
+            : []
 
         for (const filePath of Object.keys(compilation.assets)) {
           const path = filePath.replace(/\\/g, '/')
@@ -58,7 +70,10 @@ export default class BuildManifestPlugin {
             }
           }
 
-        assetMap.pages[`/${pagePath.replace(/\\/g, '/')}`] = [...filesForEntry, ...mainJsFiles]
+          assetMap.pages[`/${pagePath.replace(/\\/g, '/')}`] = [
+            ...filesForEntry,
+            ...mainJsFiles
+          ]
         }
 
         if (typeof assetMap.pages['/index'] !== 'undefined') {
@@ -67,10 +82,14 @@ export default class BuildManifestPlugin {
 
         assetMap.pages = Object.keys(assetMap.pages)
           .sort()
-        .reduce((a, c) => Object.assign(a, { [c]: assetMap.pages[c] }), {})
+          // eslint-disable-next-line
+          .reduce((a, c) => ((a[c] = assetMap.pages[c]), a), {})
 
-      compilation.assets[BUILD_MANIFEST] = new RawSource(JSON.stringify(assetMap, null, 2))
+        compilation.assets[BUILD_MANIFEST] = new RawSource(
+          JSON.stringify(assetMap, null, 2)
+        )
         callback()
-    })
+      }
+    )
   }
 }