fix(watcher): use correct file sha in url query string

Use the most recently computed file sha present on the served file
when setting the file sha in the scripts tags added to the context.html.

This fixes an issue where the sha query arg would always be the sha of
the file when karma initially started and would not take into account
changes to the file when the watcher is running.

Closes #2317, #2264

lib/middleware/karma.js

@@ -11,6 +11,8 @@
  * - setting propert caching headers
  */
 
+var _ = require('lodash')
+var from = require('core-js/library/fn/array/from')
 var path = require('path')
 var util = require('util')
 var url = require('url')
@@ -74,6 +76,12 @@ var isFirefox = function (req) {
   return firefox
 }
 
+var findByPath = function (files, path) {
+  return _.find(from(files), function (file) {
+    return file.path === path
+  })
+}
+
 var createKarmaMiddleware = function (
   filesPromise,
   serveStaticFile,
@@ -183,7 +191,12 @@ var createKarmaMiddleware = function (
               filePath = filePathToUrlPath(filePath, basePath, urlRoot, proxyPath)
 
               if (requestUrl === '/context.html') {
-                filePath += '?' + file.sha
+                var servedFile = findByPath(files.served, file.path)
+                if (servedFile) {
+                  filePath += '?' + servedFile.sha
+                } else {
+                  filePath += '?' + file.sha
+                }
               }
             }