Add support for highlights notification

pkg/analyzer_plugin/doc/tutorial/outline.md

 # Providing Outlines
 
 Outline information is used by clients to allow users to see the structure of
 their code.
 
 ## Implementation details
 
 Outline information is available through a subscription. If the server has
 subscribed for outline information in some set of files, then the plugin should
 send the information in an `analysis.outline` notification whenever the
 information needs to be updated.
 
-When a notification needs to be sent, the method`sendOutlineNotification` will
+When a notification needs to be sent, the method `sendOutlineNotification` will
 be invoked. This method is responsible for sending the notification.
 
 The easiest way to add support for this notification is by adding the classes
 `OutlineMixin` and `DartOutlineMixin` (from
 `package:analyzer_plugin/plugin/outline_mixin.dart`) to the list of mixins
 for your subclass of `ServerPlugin`. This will leave you with one abstract
 method that you need to implement: `getOutlineContributors`. That method is
 responsible for returning a list of `OutlineContributor`s. It is the outline
 contributors that produce the actual outline information. (Most plugins will
 only need a single outline contributor.)
@@ skipping 47 matching lines @@
 ```dart
 class MyPlugin extends ServerPlugin with OutlineMixin, DartOutlineMixin {
   // ...
 
   @override
   List<OutlineContributor> getOutlineContributors(String path) {
     return <OutlineContributor>[new MyOutlineContributor()];
   }
 }
 ```