Small flow tweaks in metro api

Reviewed By: jeanlauliac

Differential Revision: D6666057

fbshipit-source-id: 6f0b56fcb531e8c7a10c782342291b35399cac2b

packages/metro/src/Config.js

@@ -27,7 +27,7 @@ import type {TransformVariants} from './ModuleGraph/types.flow';
 import type {HasteImpl} from './node-haste/Module';
 import type {IncomingMessage, ServerResponse} from 'http';
 
-type Middleware = (IncomingMessage, ServerResponse, ?(?Error) => void) => void;
+type Middleware = (IncomingMessage, ServerResponse, ?() => mixed) => mixed;
 
 export type ConfigT = {
   assetRegistryPath: string,

packages/metro/src/Server/index.js

@@ -390,11 +390,11 @@ class Server {
     return JSON.stringify(Object.assign({}, options, ignoredParams));
   }
 
-  async processRequest(
+  processRequest = async (
     req: IncomingMessage,
     res: ServerResponse,
-    next?: () => mixed,
-  ) {
+    next: ?() => mixed,
+  ) => {
     const urlObj = url.parse(req.url, true);
     const {host} = req.headers;
     debug(`Handling request: ${host ? 'http://' + host : ''}${req.url}`);
@@ -421,7 +421,7 @@ class Server {
       res.writeHead(404);
       res.end();
     }
-  }
+  };
 
   _prepareDeltaBundler(
     req: IncomingMessage,

packages/metro/src/index.js

@@ -136,7 +136,6 @@ type CreateConnectMiddlewareOptions = {|
 exports.createConnectMiddleware = async function(
   options: CreateConnectMiddlewareOptions,
 ) {
-  // $FlowFixMe I don't know why Flow thinks there's an error here... runMetro IS async
   const metroServer = await runMetro({
     config: options.config,
     maxWorkers: options.maxWorkers,
@@ -150,10 +149,7 @@ exports.createConnectMiddleware = async function(
     : Config.DEFAULT;
 
   return {
-    middleware: normalizedConfig.enhanceMiddleware(
-      (req: IncomingMessage, res: ServerResponse, next: ?(?Error) => void) =>
-        metroServer.processRequest(req, res, next),
-    ),
+    middleware: normalizedConfig.enhanceMiddleware(metroServer.processRequest),
     end() {
       metroServer.end();
     },