Start on StyleManager

This will take over raw stylesheet management from the theme manager
now that it’s becoming more complex with the need to target specific
host elements. Instead of actually adding nodes to the head of the
document, it will instead simply manage a set of stylesheets we want to
apply and leave actual DOM manipulation to <atom-styles> custom elements
that can render the set of active stylesheets in the appropriate
locations.

spec/style-manager-spec.coffee

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+StyleManager = require '../src/style-manager'
+
+describe "StyleManager", ->
+  manager = null
+
+  beforeEach ->
+    manager = new StyleManager
+
+  describe "::addStyleSheet(source, params)", ->
+    it "adds a stylesheet based on the given source and returns a disposable allowing it to be removed", ->
+      addEvents = []
+      removeEvents = []
+      manager.onDidAddStyleSheet (event) -> addEvents.push(event)
+      manager.onDidRemoveStyleSheet (event) -> removeEvents.push(event)
+
+      disposable = manager.addStyleSheet("a {color: red;}")
+
+      expect(addEvents.length).toBe 1
+      expect(addEvents[0].styleElement.textContent).toBe "a {color: red;}"
+
+      styleElements = manager.getStyleElements()
+      expect(styleElements.length).toBe 1
+      expect(styleElements[0].textContent).toBe "a {color: red;}"
+
+      disposable.dispose()
+
+      expect(removeEvents.length).toBe 1
+      expect(removeEvents[0].styleElement.textContent).toBe "a {color: red;}"
+      expect(manager.getStyleElements().length).toBe 0

src/style-manager.coffee

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+{Emitter, Disposable} = require 'event-kit'
+
+module.exports =
+class StyleManager
+  constructor: ->
+    @emitter = new Emitter
+    @styleElements = []
+
+  onDidAddStyleSheet: (callback) ->
+    @emitter.on 'did-add-style-sheet', callback
+
+  onDidRemoveStyleSheet: (callback) ->
+    @emitter.on 'did-remove-style-sheet', callback
+
+  getStyleElements: ->
+    @styleElements.slice()
+
+  addStyleSheet: (source) ->
+    styleElement = document.createElement('style')
+    styleElement.textContent = source
+    @styleElements.push(styleElement)
+    @emitter.emit 'did-add-style-sheet', {styleElement}
+
+    new Disposable => @removeStyleElement(styleElement)
+
+  removeStyleElement: (styleElement) ->
+    index = @styleElements.indexOf(styleElement)
+    unless index is -1
+      @styleElements.splice(index, 1)
+      @emitter.emit 'did-remove-style-sheet', {styleElement}