WIP: Destroy nested tab stops when engulfed by a buffer change

Has 2 failing specs...

There are still some issue with this code's interaction with the undo
system. The tab stops will need to be or destroyed when certain
changes are undone or redone.

src/app/anchor-range.coffee

@@ -1,4 +1,7 @@
 Range = require 'range'
+EventEmitter = require 'event-emitter'
+Subscriber = require 'subscriber'
+_ = require 'underscore'
 
 module.exports =
 class AnchorRange
@@ -6,11 +9,14 @@ class AnchorRange
   end: null
   buffer: null
   editSession: null # optional
+  destroyed: false
 
   constructor: (bufferRange, @buffer, @editSession) ->
     bufferRange = Range.fromObject(bufferRange)
     @startAnchor = @buffer.addAnchorAtPosition(bufferRange.start, ignoreChangesStartingOnAnchor: true)
     @endAnchor = @buffer.addAnchorAtPosition(bufferRange.end)
+    @subscribe @startAnchor, 'destroyed', => @destroy()
+    @subscribe @endAnchor, 'destroyed', => @destroy()
 
   getBufferRange: ->
     new Range(@startAnchor.getBufferPosition(), @endAnchor.getBufferPosition())
@@ -22,7 +28,14 @@ class AnchorRange
     @getBufferRange().containsPoint(bufferPosition)
 
   destroy: ->
+    return if @destroyed
+    @unsubscribe()
     @startAnchor.destroy()
     @endAnchor.destroy()
     @buffer.removeAnchorRange(this)
     @editSession?.removeAnchorRange(this)
+    @destroyed = true
+    @trigger 'destroyed'
+
+_.extend(AnchorRange.prototype, EventEmitter)
+_.extend(AnchorRange.prototype, Subscriber)

src/app/anchor.coffee

@@ -10,6 +10,7 @@ class Anchor
   screenPosition: null
   ignoreChangesStartingOnAnchor: false
   strong: false
+  destroyed: false
 
   constructor: (@buffer, options = {}) ->
     { @editSession, @ignoreChangesStartingOnAnchor, @strong } = options
@@ -81,8 +82,10 @@ class Anchor
     @setScreenPosition(screenPosition, bufferChange: options.bufferChange, clip: false, assignBufferPosition: false, autoscroll: options.autoscroll)
 
   destroy: ->
+    return if @destroyed
     @buffer.removeAnchor(this)
     @editSession?.removeAnchor(this)
+    @destroyed = true
     @trigger 'destroyed'
 
 _.extend(Anchor.prototype, EventEmitter)