docs(cheatsheet): fix Dart cheatsheet

Also deletes an extraneous period from the JS cheatsheet.

Closes #5936

modules/angular2/docs/cheatsheet/built-in-directives.md

@@ -4,7 +4,7 @@ Built-in directives
 @description
 {@target ts}`import {NgIf, ...} from 'angular2/common';`{@endtarget}
 {@target js}Available from the `ng.common` namespace{@endtarget}
-{@target dart}`import 'package:angular2/common.dart';`{@endtarget}
+{@target dart}Available using `platform_directives` in pubspec{@endtarget}
 
 @cheatsheetItem
 syntax: