Add plugin support for occurrences (and fix typo)

pkg/analyzer_plugin/doc/tutorial/highlights.md

 # Providing Highlighting Information
 
 Highlighting information is used by clients to help users identify different
 syntactic and semantic regions of their code.
 
 Syntactic highlighting is highlighting that is based completely on the syntax of
 the code. For example, editors will often provide unique colors for comments,
 string literals, and numeric literals.
 
 Semantic highlighting is highlighting that is based on semantic information. For
@@ skipping 50 matching lines @@
     }
   }
 }
 
 class HighlightsVisitor extends RecursiveAstVisitor {
   final HighlightsCollector collector;
 
   HighlightsVisitor(this.collector);
 
   @override
-  void visitSimpleIdentifier(ClassDeclaration node) {
+  void visitSimpleIdentifier(SimpleIdentifier node) {
     // ...
   }
 }
 ```
 
 Given a contributor like the one above, you can implement your plugin similar to
 the following:
 
 ```dart
 class MyPlugin extends ServerPlugin with HighlightsMixin, DartHighlightsMixin {
   // ...
 
   @override
   List<HighlightsContributor> getHighlightsContributors(String path) {
     return <HighlightsContributor>[new MyHighlightsContributor()];
   }
 }
 ```