Print assets sizes after build (#5664)

It looks like this : 

![capture d ecran 2018-11-13 a 00 47 12](https://user-images.githubusercontent.com/6616955/48361936-7cc13d80-e6dd-11e8-8aa8-c5a73beae966.png)

I'm not sure about the License part, can you check @timneutkens ?

packages/next/build/webpack.js

@@ -20,6 +20,7 @@ import {SERVER_DIRECTORY, REACT_LOADABLE_MANIFEST, CLIENT_STATIC_FILES_RUNTIME_W
 import {NEXT_PROJECT_ROOT, NEXT_PROJECT_ROOT_NODE_MODULES, NEXT_PROJECT_ROOT_DIST, DEFAULT_PAGES_DIR} from '../lib/constants'
 import AutoDllPlugin from 'autodll-webpack-plugin'
 import TerserPlugin from 'terser-webpack-plugin'
+import AssetsSizePlugin from './webpack/plugins/assets-size-plugin'
 
 // The externals config makes sure that
 // on the server side when modules are
@@ -293,7 +294,8 @@ export default async function getBaseWebpackConfig (dir: string, {dev = false, i
       !isServer && new BuildManifestPlugin(),
       !isServer && new PagesPlugin(),
       isServer && new NextJsSsrImportPlugin(),
-      isServer && new NextJsSSRModuleCachePlugin({outputPath})
+      isServer && new NextJsSSRModuleCachePlugin({outputPath}),
+      !isServer && !dev && new AssetsSizePlugin({buildId, distDir})
     ].filter(Boolean)
   }
 

packages/next/build/webpack/plugins/assets-size-plugin.js

@@ -0,0 +1,115 @@
+// Some parts of this file are licensed under the following :
+/**
+MIT License
+
+Copyright (c) Sindre Sorhus <sindresorhus@gmail.com> (sindresorhus.com)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+*/
+
+import { gzip as _gzip } from 'zlib'
+import { relative as pathRelative } from 'path'
+import promisify from '../../../lib/promisify'
+import {
+  IS_BUNDLED_PAGE_REGEX,
+  BUILD_MANIFEST,
+  REACT_LOADABLE_MANIFEST
+} from 'next-server/constants'
+
+const gzip = promisify(_gzip)
+
+// This part is based on https://github.com/sindresorhus/pretty-bytes/blob/v5.1.0/index.js
+// It's been edited for the needs of this script
+// See the LICENSE at the top of the file
+const UNITS = ['B', 'kB', 'MB']
+const prettyBytes = number => {
+  const exponent = Math.min(
+    Math.floor(Math.log10(number) / 3),
+    UNITS.length - 1
+  )
+  number = Number((number / Math.pow(1000, exponent)).toPrecision(3))
+  const unit = UNITS[exponent]
+  return number + ' ' + unit
+}
+
+export default class AssetsSizePlugin {
+  constructor ({ buildId, distDir }) {
+    this.buildId = buildId
+    this.distDir = distDir ? pathRelative(process.cwd(), distDir) + '/' : ''
+  }
+
+  formatFilename (rawFilename) {
+    // add distDir
+    let filename = this.distDir + rawFilename
+
+    // shorten buildId
+    if (this.buildId) {
+      filename = filename.replace(
+        this.buildId + '/',
+        this.buildId.substring(0, 4) + '****/'
+      )
+    }
+
+    // shorten hashes
+    filename = filename.replace(
+      /(.*[-.])([0-9a-f]{8,})(\.js|\.css)/,
+      (_, c1, hash, c2) => c1 + hash.substring(0, 4) + '****' + c2
+    )
+
+    return filename
+  }
+
+  async printAssetsSize (assets) {
+    const sizes = await Promise.all(
+      Object.keys(assets)
+        .filter(
+          filename =>
+            filename !== REACT_LOADABLE_MANIFEST && filename !== BUILD_MANIFEST
+        )
+        .sort((a, b) => {
+          // put pages at the top, then the rest
+          const [pa, pb] = [a, b].map(x => IS_BUNDLED_PAGE_REGEX.exec(x))
+          if (pa && !pb) return -1
+          if (pb && !pa) return 1
+          if (a > b) return 1
+          return -1
+        })
+        .map(async filename => {
+          const asset = assets[filename]
+          const size = (await gzip(asset.source())).length
+
+          return {
+            filename,
+            prettySize: prettyBytes(size)
+          }
+        })
+    )
+
+    // find longest prettySize string size
+    const longestPrettySize = Math.max(
+      ...sizes.map(({ prettySize }) => prettySize.length)
+    )
+
+    let message = '\nBrowser assets sizes after gzip:\n\n'
+
+    for (let { filename, prettySize } of sizes) {
+      const padding = ' '.repeat(longestPrettySize - prettySize.length)
+      const formattedSize = prettySize
+      const formattedFilename = this.formatFilename(filename)
+
+      message += `   ${padding}${formattedSize}  ${formattedFilename}\n`
+    }
+
+    console.log(message)
+  }
+
+  async apply (compiler) {
+    compiler.hooks.afterEmit.tapPromise('AssetsSizePlugin', compilation =>
+      this.printAssetsSize(compilation.assets).catch(console.error)
+    )
+  }
+}