Add DartEditor.app latest dev channel

Based on darteditor.rb from the main repo.

Casks/darteditor-dev.rb

@@ -0,0 +1,12 @@
+cask :v1 => 'darteditor-dev' do
+  version :latest
+  sha256 :no_check
+
+  # googleapis.com is the official download host per the vendor homepage
+  url 'https://storage.googleapis.com/dart-archive/channels/dev/release/latest/editor/darteditor-macos-x64.zip'
+  name 'Dart Editor'
+  homepage 'https://www.dartlang.org/tools/editor/'
+  license :bsd
+
+  app 'dart/DartEditor.app', :target => 'DartEditor-dev.app'
+end