Buffers retain path when file is deleted and can be re-saved

Path watching resumes once the file is saved again. This commit allows files to be created for as-yet nonexistent paths. We won't call `$native.watchPath` until we have at least 1 subscription to the file in JS and the file exists on disk.

Also, we moved execution of the path watcher callbacks until after the callbacks data structure is updated in order to avoid confusing behavior in specs.

src/app/buffer.coffee

@@ -94,7 +94,7 @@ class Buffer
 
     @file?.off()
     @file = new File(path)
-    @subscribeToFile()
+    @subscribeToFile() if @file.exists()
 
     @trigger "path-change", this
 
@@ -236,14 +236,18 @@ class Buffer
     @saveAs(@getPath())
 
   saveAs: (path) ->
-    if not path then throw new Error("Can't save buffer with no file path")
+    unless path then throw new Error("Can't save buffer with no file path")
 
     @trigger 'before-save'
-    @cachedDiskContents = @getText()
-    fs.write path, @cachedDiskContents
+
     @file?.updateMd5()
     @setPath(path)
 
+    text = @getText()
+
+    @cachedDiskContents = text
+    @file.write(text)
+    @subscribeToFile()
     @trigger 'after-save'
 
   isModified: ->
@@ -389,4 +393,7 @@ class Buffer
       @trigger 'stopped-changing'
     @stoppedChangingTimeout = setTimeout(stoppedChangingCallback, @stoppedChangingDelay)
 
+  fileExists: ->
+    @file.exists()
+
 _.extend(Buffer.prototype, EventEmitter)

src/app/file.coffee

@@ -9,8 +9,10 @@ class File
   md5: null
 
   constructor: (@path) ->
-    throw "Creating file with path that is not a file: #{@path}" unless fs.isFile(@path)
-    @updateMd5()
+    if @exists() and not fs.isFile(@path)
+      throw new Error(@path + " is a directory")
+
+    @updateMd5() if @exists()
 
   setPath: (@path) ->
 
@@ -19,6 +21,12 @@ class File
   getBaseName: ->
     fs.base(@path)
 
+  write: (text) ->
+    previouslyExisted = @exists()
+    fs.write(@getPath(), text)
+    @updateMd5()
+    @subscribeToNativeChangeEvents() if not previouslyExisted and @subscriptionCount() > 0
+
   exists: ->
     fs.exists(@getPath())
 
@@ -26,7 +34,7 @@ class File
     @md5 = fs.md5ForPath(@path)
 
   afterSubscribe: ->
-    @subscribeToNativeChangeEvents() if @subscriptionCount() == 1
+    @subscribeToNativeChangeEvents() if @exists() and @subscriptionCount() == 1
 
   afterUnsubscribe: ->
     @unsubscribeFromNativeChangeEvents() if @subscriptionCount() == 0
@@ -52,8 +60,8 @@ class File
       @subscribeToNativeChangeEvents()
       @handleNativeChangeEvent("contents-change", @getPath())
     else
+      @unsubscribeFromNativeChangeEvents()
       @trigger "remove"
-      @off()
 
   subscribeToNativeChangeEvents: ->
     @watchId = $native.watchPath @path, (eventType, path) =>