Add selector parser helper class

src/command-registry.coffee

@@ -2,6 +2,7 @@
 {specificity} = require 'clear-cut'
 _ = require 'underscore-plus'
 {$} = require './space-pen-extensions'
+{isSelectorValid} = require './selector-parser'
 
 SequenceCount = 0
 SpecificityCache = {}
@@ -87,7 +88,7 @@ class CommandRegistry
       return disposable
 
     if typeof target is 'string'
-      unless @isSelectorValid(target)
+      unless isSelectorValid(target)
         throw new Error("'#{target}' is not a valid selector")
       @addSelectorBasedListener(target, commandName, callback)
     else
@@ -237,20 +238,6 @@ class CommandRegistry
       window.addEventListener(commandName, @handleCommandEvent, true)
       @registeredCommands[commandName] = true
 
-  isSelectorValid: (selector) ->
-    @selectorCache ?= {}
-    cachedValue = @selectorCache[selector]
-    return cachedValue if cachedValue?
-
-    @testElement ?= document.createElement('div')
-    try
-      @testElement.webkitMatchesSelector(selector)
-      @selectorCache[selector] = true
-      true
-    catch selectorError
-      @selectorCache[selector] = false
-      false
-
 class SelectorBasedListener
   constructor: (@selector, @callback) ->
     @specificity = (SpecificityCache[@selector] ?= specificity(@selector))

src/selector-parser.coffee

@@ -0,0 +1,16 @@
+selectorCache = null
+testElement = null
+
+exports.isSelectorValid = (selector) ->
+  selectorCache ?= {}
+  cachedValue = selectorCache[selector]
+  return cachedValue if cachedValue?
+
+  testElement ?= document.createElement('div')
+  try
+    testElement.webkitMatchesSelector(selector)
+    selectorCache[selector] = true
+    true
+  catch selectorError
+    selectorCache[selector] = false
+    false