Re-tokenize buffer when its grammar is updated

This can happen if a grammar that the grammar includes is added or
removed from the syntax global.

spec/app/tokenized-buffer-spec.coffee

@@ -328,6 +328,26 @@ describe "TokenizedBuffer", ->
 
         expect(tokenizedBuffer.lineForScreenRow(2).text).toBe "#{tabAsSpaces} buy()#{tabAsSpaces}while supply > demand"
 
+  describe "when the language mode emits a 'grammar-updated' event based on an included grammar being activated", ->
+    it "retokenizes the buffer", ->
+      atom.activatePackage('ruby.tmbundle', sync: true)
+      atom.activatePackage('ruby-on-rails-tmbundle', sync: true)
+
+      editSession = project.buildEditSession()
+      editSession.setVisible(true)
+      editSession.setGrammar(syntax.selectGrammar('test.erb'))
+      editSession.buffer.setText("<div class='name'><%= User.find(2).full_name %></div>")
+      tokenizedBuffer = editSession.displayBuffer.tokenizedBuffer
+      fullyTokenize(tokenizedBuffer)
+
+      {tokens} = tokenizedBuffer.lineForScreenRow(0)
+      expect(tokens[0]).toEqual value: "<div class='name'>", scopes: ["text.html.ruby"]
+
+      atom.activatePackage('html.tmbundle', sync: true)
+      fullyTokenize(tokenizedBuffer)
+      {tokens} = tokenizedBuffer.lineForScreenRow(0)
+      expect(tokens[0]).toEqual value: '<', scopes: ["text.html.ruby","meta.tag.block.any.html","punctuation.definition.tag.begin.html"]
+
   describe "when a Git commit message file is tokenized", ->
     beforeEach ->
       atom.activatePackage('git.tmbundle', sync: true)

src/app/language-mode.coffee

@@ -34,6 +34,7 @@ class LanguageMode
     return if grammar is @grammar
     @grammar = grammar
     @currentGrammarScore = score ? grammar.getScore(@buffer.getPath(), @buffer.getText())
+    @subscribe @grammar, 'grammar-updated', => @trigger 'grammar-updated'
     @trigger 'grammar-changed', grammar
 
   reloadGrammar: ->

src/app/null-grammar.coffee

@@ -1,4 +1,6 @@
 Token = require 'token'
+EventEmitter = require 'event-emitter'
+_ = require 'underscore'
 
 ###
 # Internal #
@@ -14,3 +16,5 @@ class NullGrammar
     { tokens: [new Token(value: line, scopes: ['null-grammar.text.plain'])] }
 
   grammarAddedOrRemoved: -> # no op
+
+_.extend NullGrammar.prototype, EventEmitter

src/app/tokenized-buffer.coffee

@@ -28,6 +28,7 @@ class TokenizedBuffer
     @resetScreenLines()
     @buffer.on "changed.tokenized-buffer#{@id}", (e) => @handleBufferChange(e)
     @languageMode.on 'grammar-changed', => @resetScreenLines()
+    @languageMode.on 'grammar-updated', => @resetScreenLines()
 
   resetScreenLines: ->
     @screenLines = @buildPlaceholderScreenLinesForRows(0, @buffer.getLastRow())