fix: Promise.prototype.finally is object (#16620)

The [finally-polyfill](https://gist.github.com/developit/d970bac18430943e4b3392b029a2a96c)  did not expose any values.
![image](https://user-images.githubusercontent.com/15967520/91409583-2f916a00-e878-11ea-86d0-189966ceb83c.png)

Author: tengmaoqing

packages/next/client/index.tsx

@@ -21,6 +21,8 @@ import PageLoader, { looseToArray, StyleSheetTuple } from './page-loader'
 import measureWebVitals from './performance-relayer'
 import { createRouter, makePublicRouterInstance } from './router'
 
+require('next/dist/build/polyfills/finally-polyfill.min')
+
 /// <reference types="react-dom/experimental" />
 
 declare let __webpack_public_path__: string
@@ -41,10 +43,6 @@ declare global {
 type RenderRouteInfo = PrivateRouteInfo & { App: AppComponent }
 type RenderErrorProps = Omit<RenderRouteInfo, 'Component' | 'styleSheets'>
 
-if (!('finally' in Promise.prototype)) {
-  ;(Promise.prototype as PromiseConstructor['prototype']).finally = require('next/dist/build/polyfills/finally-polyfill.min')
-}
-
 const data: typeof window['__NEXT_DATA__'] = JSON.parse(
   document.getElementById('__NEXT_DATA__')!.textContent!
 )

packages/next/package.json

@@ -181,7 +181,7 @@
     "escape-string-regexp": "2.0.0",
     "etag": "1.8.1",
     "file-loader": "6.0.0",
-    "finally-polyfill": "0.1.0",
+    "finally-polyfill": "0.2.0",
     "find-up": "4.1.0",
     "fresh": "0.5.2",
     "gzip-size": "5.1.1",