Subscribe to <atom-styles> element to shim events in ThemeManager

This is better than subscribing to atom.styles because the events it
emits contain the *actual* style elements we insert into the DOM.

src/atom.coffee

@@ -204,6 +204,7 @@ class Atom extends Model
     @commands = new CommandRegistry
     @packages = new PackageManager({devMode, configDirPath, resourcePath, safeMode})
     @styles = new StyleManager
+    document.head.appendChild(new StylesElement)
     @themes = new ThemeManager({packageManager: @packages, configDirPath, resourcePath, safeMode})
     @contextMenu = new ContextMenuManager({resourcePath, devMode})
     @menu = new MenuManager({resourcePath})
@@ -222,8 +223,6 @@ class Atom extends Model
 
     @windowEventHandler = new WindowEventHandler
 
-    document.head.appendChild(new StylesElement)
-
   ###
   Section: Event Subscription
   ###

src/theme-manager.coffee

@@ -24,26 +24,24 @@ class ThemeManager
     @initialLoadComplete = false
     @packageManager.registerPackageActivator(this, ['theme'])
 
-    atom.styles.onDidAddStyleElement @styleElementAdded.bind(this)
-    atom.styles.onDidRemoveStyleElement @styleElementRemoved.bind(this)
-    atom.styles.onDidUpdateStyleElement @styleElementUpdated.bind(this)
+    stylesElement = document.head.querySelector('atom-styles')
+    stylesElement.onDidAddStyleElement @styleElementAdded.bind(this)
+    stylesElement.onDidRemoveStyleElement @styleElementRemoved.bind(this)
+    stylesElement.onDidUpdateStyleElement @styleElementUpdated.bind(this)
 
-  styleElementAdded: (element) ->
-    sheet = @styleSheetForElement(element)
+  styleElementAdded: ({sheet}) ->
     @emit 'stylesheet-added', sheet
     @emitter.emit 'did-add-stylesheet', sheet
     @emit 'stylesheets-changed'
     @emitter.emit 'did-change-stylesheets'
 
-  styleElementRemoved: (element) ->
-    sheet = @styleSheetForElement(element)
+  styleElementRemoved: ({sheet}) ->
     @emit 'stylesheet-removed', sheet
     @emitter.emit 'did-remove-stylesheet', sheet
     @emit 'stylesheets-changed'
     @emitter.emit 'did-change-stylesheets'
 
-  styleElementUpdated: (element) ->
-    sheet = @styleSheetForElement(element)
+  styleElementUpdated: ({sheet}) ->
     @emit 'stylesheet-removed', sheet
     @emitter.emit 'did-remove-stylesheet', sheet
     @emit 'stylesheet-added', sheet
@@ -51,10 +49,6 @@ class ThemeManager
     @emit 'stylesheets-changed'
     @emitter.emit 'did-change-stylesheets'
 
-  styleSheetForElement: (element) ->
-    @stylesElement ?= document.head.querySelector('atom-styles')
-    @stylesElement.styleElementClonesByOriginalElement.get(element)?.sheet
-
   ###
   Section: Event Subscription
   ###