Fix .json import error

This is a regression we've added by vercel#926 (tree-shaking-support)

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,