Dart.gitignore updated to reflect instructions provided by authors

Dart.gitignore

-.DS_Store
-packages
+// Don’t commit the following files and directories created by pub, Dart Editor, and dart2js
+packages/
 .project
+.buildlog
+*.js_
+*.js.deps
+*.js.map
+
+// Include when developing application packages
 pubspec.lock 
+
+// Avoid committing generated JavaScript files
+*.dart.js
+
+// Check for any other incovenient files generated by OS 
+// and other tools such as Eclipse, IntelliJ, etc.