remove useless boolean since calling resolve multiple times is fine (close vuejs#177)

Author: yyx990803

build/setup-dev-server.js

@@ -7,10 +7,9 @@ const serverConfig = require('./webpack.server.config')
 module.exports = function setupDevServer (app, cb) {
  let bundle, clientManifest
  let resolve
- let resolved = false
  const readyPromise = new Promise(r => { resolve = r })
  const ready = (...args) => {
- if (!resolved) resolve()
+ resolve()
  cb(...args)
  }