Move toExistOnDisk matcher to spec helper

spec/spec-helper.coffee

@@ -137,6 +137,11 @@ addCustomMatchers = (spec) ->
       this.message = => "Expected object with length #{@actual.length} to#{notText} have length #{expected}"
       @actual.length == expected
 
+    toExistOnDisk: (expected) ->
+      notText = this.isNot and " not" or ""
+      @message = -> return "Expected path '" + @actual + "'" + notText + " to exist."
+      fs.exists(@actual)
+
 window.keyIdentifierForKey = (key) ->
   if key.length > 1 # named key
     key

src/packages/package-generator/spec/package-generator-spec.coffee

@@ -37,12 +37,6 @@ describe 'Package Generator', ->
       packagePath = "/tmp/atom-packages/#{packageName}"
       fs.remove(packagePath) if fs.exists(packagePath)
 
-      @addMatchers
-        toExistOnDisk: (expected) ->
-          notText = this.isNot and " not" or ""
-          @message = -> return "Expected path '" + @actual + "'" + notText + " to exist."
-          fs.exists(@actual)
-
     afterEach ->
       fs.remove(packagePath) if fs.exists(packagePath)