Find custom babel config location properly. (#969)

* Find custom babel config location properly.
Earlier we simply check for the .bablerc file in the dir.
But the actual logic is much complex.
Now we are using the babel's actual logic to find the
custom config location.

* Fix failing tests.

examples/.babelrc

 {
+    "presets": ["../babel"]
 }
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server/build/babel/find-config-location.js

+import { join } from 'path'
+import buildConfigChain from 'babel-core/lib/transformation/file/options/build-config-chain'
+
+export default function findBabelConfigLocation (dir) {
+  // We need to provide a location of a filename inside the `dir`.
+  // For the name of the file, we could be provide anything.
+  const filename = join(dir, 'filename.js')
+  const options = { babelrc: true, filename }
+
+  // First We need to build the config chain.
+  // Then we need to remove the config item with the location as "base".
+  // That's the config we are passing as the "options" below
+  const configList = buildConfigChain(options).filter(i => i.loc !== 'base')
+
+  return configList[0] ? configList[0].loc : null
+}

server/build/webpack.js

 import { resolve, join } from 'path'
 import { createHash } from 'crypto'
-import { existsSync } from 'fs'
 import webpack from 'webpack'
 import glob from 'glob-promise'
 import WriteFilePlugin from 'write-file-webpack-plugin'
@@ -11,6 +10,7 @@ import WatchPagesPlugin from './plugins/watch-pages-plugin'
 import JsonPagesPlugin from './plugins/json-pages-plugin'
 import getConfig from '../config'
 import * as babelCore from 'babel-core'
+import findBabelConfigLocation from './babel/find-config-location'
 
 const documentPage = join('pages', '_document.js')
 const defaultPages = [
@@ -115,9 +115,10 @@ export default async function createCompiler (dir, { dev = false, quiet = false
     presets: []
   }
 
-  const hasBabelRc = existsSync(join(dir, '.babelrc'))
-  if (hasBabelRc) {
-    console.log('> Using .babelrc defined in your app root')
+  const configLocation = findBabelConfigLocation(dir)
+  if (configLocation) {
+    console.log(`> Using external babel configuration`)
+    console.log(`> location: "${configLocation}"`)
   } else {
     mainBabelOptions.presets.push(require.resolve('./babel/preset'))
   }

test/.babelrc

+{
+  "presets": [
+    // To let test apps(and Jest) to use Next's babel preset
+    "../babel",
+    // To transpile import statements into commonjs.
+    // That's because Jest(runs in Node.js) don't know how to handle them.
+    "es2015"
+   ]
+}
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