Fix .json import issue (#944)

* Fix .json import error
This is a regression we've added by #926 (tree-shaking-support)

* Add a test case.

server/build/loaders/emit-file-loader.js

@@ -16,7 +16,7 @@ module.exports = function (content, sourceMap) {
   }
 
   if (query.transform) {
-    const transformed = query.transform({ content, sourceMap })
+    const transformed = query.transform({ content, sourceMap, interpolatedName })
     return emit(transformed.content, transformed.sourceMap)
   }
 

server/build/webpack.js

@@ -149,7 +149,12 @@ export default async function createCompiler (dir, { dev = false, quiet = false
       // By default, our babel config does not transpile ES2015 module syntax because
       // webpack knows how to handle them. (That's how it can do tree-shaking)
       // But Node.js doesn't know how to handle them. So, we have to transpile them here.
-      transform ({ content, sourceMap }) {
+      transform ({ content, sourceMap, interpolatedName }) {
+        // Only handle .js files
+        if (!(/\.js$/.test(interpolatedName))) {
+          return { content, sourceMap }
+        }
+
         const transpiled = babelCore.transform(content, {
           presets: ['es2015'],
           sourceMaps: dev ? 'both' : false,

test/integration/basic/lib/data.json

@@ -0,0 +1,3 @@
+{
+    "name": "Zeit"
+}

test/integration/basic/pages/json.js

@@ -0,0 +1,5 @@
+import data from '../lib/data'
+
+export default () => (
+  <div>{data.name}</div>
+)

test/integration/basic/test/rendering.js

@@ -61,6 +61,11 @@ export default function ({ app }, suiteName, render) {
       expect($('pre').text().includes(expectedErrorMessage)).toBeTruthy()
     })
 
+    test('allows to import .json files', async () => {
+      const html = await render('/json')
+      expect(html.includes('Zeit')).toBeTruthy()
+    })
+
     test('error', async () => {
       const $ = await get$('/error')
       expect($('pre').text()).toMatch(/This is an expected error/)