refactor(filelist): rename promise -> lastCompletedRefresh and remove…

… unused promise (#3060)

lib/file-list.js

@@ -29,7 +29,8 @@ class FileList {
 
     this.buckets = new Map()
 
-    this._refreshing = Promise.resolve()
+    // A promise that is pending if and only if we are active in this.refresh_()
+    this._refreshing = null
 
     const emit = () => {
       this._emitter.emit('file_list_modified', this.files)
@@ -65,8 +66,8 @@ class FileList {
   _refresh () {
     const matchedFiles = new Set()
 
-    let promise
-    promise = Promise.map(this._patterns, (patternObject) => {
+    let lastCompletedRefresh = this._refreshing
+    lastCompletedRefresh = Promise.map(this._patterns, (patternObject) => {
       const pattern = patternObject.pattern
       const type = patternObject.type
 
@@ -113,14 +114,18 @@ class FileList {
         })
     })
       .then(() => {
-        if (this._refreshing !== promise) {
+        // When we return from this function the file processing chain will be
+        // complete. In the case of two fast refresh() calls, the second call
+        // will overwrite this._refreshing, and we want the status to reflect
+        // the second call and skip the modification event from the first call.
+        if (this._refreshing !== lastCompletedRefresh) {
           return this._refreshing
         }
         this._emitModified(true)
         return this.files
       })
 
-    return promise
+    return lastCompletedRefresh
   }
 
   get files () {