Add and load React production source maps

This can be reverted when a React canary version is used that
includes facebook/react#28881.

Author: unstubbable

packages/next/package.json

@@ -292,6 +292,7 @@
     "setimmediate": "1.0.5",
     "shell-quote": "1.7.3",
     "source-map": "0.6.1",
+    "source-map-loader": "5.0.0",
     "source-map08": "npm:source-map@0.8.0-beta.0",
     "stacktrace-parser": "0.1.10",
     "stream-browserify": "3.0.0",