Upgrade existing selectors after attaching atom-styles element

Style elements don’t have a .sheet property until they are actually
attached to the DOM.

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

src/styles-element.coffee

@@ -18,6 +18,9 @@ class StylesElement extends HTMLElement
     @styleElementClonesByOriginalElement = new WeakMap
 
   attachedCallback: ->
+    if @context is 'atom-text-editor'
+      for styleElement in @children
+        @upgradeDeprecatedSelectors(styleElement)
     @initialize()
 
   detachedCallback: ->
@@ -48,6 +51,7 @@ class StylesElement extends HTMLElement
     return unless @styleElementMatchesContext(styleElement)
 
     styleElementClone = styleElement.cloneNode(true)
+    styleElementClone.sourcePath = styleElement.sourcePath
     styleElementClone.context = styleElement.context
     @styleElementClonesByOriginalElement.set(styleElement, styleElementClone)
 
@@ -61,7 +65,7 @@ class StylesElement extends HTMLElement
     @insertBefore(styleElementClone, insertBefore)
 
     if @context is 'atom-text-editor'
-      @upgradeDeprecatedSelectors(styleElementClone, styleElement.sourcePath)
+      @upgradeDeprecatedSelectors(styleElementClone)
 
     @emitter.emit 'did-add-style-element', styleElementClone
 
@@ -82,7 +86,9 @@ class StylesElement extends HTMLElement
   styleElementMatchesContext: (styleElement) ->
     not @context? or styleElement.context is @context
 
-  upgradeDeprecatedSelectors: (styleElement, sourcePath) ->
+  upgradeDeprecatedSelectors: (styleElement) ->
+    return unless styleElement.sheet?
+
     upgradedSelectors = []
 
     for rule in styleElement.sheet.cssRules
@@ -99,7 +105,7 @@ class StylesElement extends HTMLElement
         upgradedSelectors.push({inputSelector, outputSelector})
 
     if upgradedSelectors.length > 0
-      warning = "Upgraded the following syntax theme selectors in `#{sourcePath}` for shadow DOM compatibility:\n\n"
+      warning = "Upgraded the following syntax theme selectors in `#{styleElement.sourcePath}` for shadow DOM compatibility:\n\n"
       for {inputSelector, outputSelector} in upgradedSelectors
         warning += "`#{inputSelector}` => `#{outputSelector}`\n"