feat(server): Add public API to force a file refresh

Adds a public API similar to `server.refreshFiles()` that allows to refresh a specific file.
This is useful for preprocessors that process files with dependencies (sass, js modules, postcss), so they can trigger an update on a file when a change is detected on a dependency.
Contrary to `server.refreshFiles()` it allows to refresh only the file that need to be processed again.

docs/dev/04-public-api.md

@@ -33,6 +33,14 @@ Trigger a file list refresh. Returns a promise.
 server.refreshFiles()
 ```
 
+### **server.refreshFile(path)**
+
+Trigger a file refresh. Returns a promise.
+
+```javascript
+server.refreshFile('src/js/module-dep.js')
+```
+
 ### Events
 
 The `server` object is an [`EventEmitter`](https://nodejs.org/docs/latest/api/events.html#events_class_events_eventemitter). You can simply listen to events like this:

lib/file-list.js

@@ -213,7 +213,7 @@ class FileList {
       })
   }
 
-  changeFile (path) {
+  changeFile (path, force) {
     const pattern = this._findIncluded(path)
     const file = this._findFile(path, pattern)
 
@@ -226,14 +226,14 @@ class FileList {
       fs.statAsync(path),
       this._refreshing
     ]).spread((stat) => {
-      if (stat.mtime <= file.mtime) throw new Promise.CancellationError()
+      if (!force && stat.mtime <= file.mtime) throw new Promise.CancellationError()
 
       file.mtime = stat.mtime
       return this._preprocess(file)
     })
       .then(() => {
         log.info('Changed file "%s".', path)
-        this._emitModified()
+        this._emitModified(force)
         return this.files
       })
       .catch(Promise.CancellationError, () => this.files)

lib/server.js

@@ -133,6 +133,10 @@ class Server extends KarmaEventEmitter {
     return this._fileList ? this._fileList.refresh() : Promise.resolve()
   }
 
+  refreshFile (path) {
+    return this._fileList ? this._fileList.changeFile(path) : Promise.resolve()
+  }
+
   _start (config, launcher, preprocess, fileList, capturedBrowsers, executor, done) {
     if (config.detached) {
       this._detach(config, done)

test/unit/file-list.spec.js

@@ -632,6 +632,23 @@ describe('FileList', () => {
       })
     })
 
+    it('fire "file_list_modified" if force is true even if mtime has not changed', () => {
+      // MATCH: /some/a.js, /some/b.js, /a.txt
+      list = new List(patterns('/some/*.js', '/a.*'), [], emitter, preprocess)
+
+      var modified = sinon.stub()
+      emitter.on('file_list_modified', modified)
+
+      return list.refresh().then((files) => {
+        // not touching the file, stat will return still the same
+        modified.reset()
+
+        return list.changeFile('/some/b.js', true).then(() => {
+          expect(modified).to.have.been.calledOnce
+        })
+      })
+    })
+
     it('does not fire "file_list_modified" if mtime has not changed', () => {
       // chokidar on fucking windows sometimes fires event multiple times
       // MATCH: /some/a.js, /some/b.js, /a.txt