Add plugin support for occurrences (and fix typo)

pkg/analyzer_plugin/doc/tutorial/highlights.md

Index: pkg/analyzer_plugin/doc/tutorial/highlights.md
diff --git a/pkg/analyzer_plugin/doc/tutorial/highlights.md b/pkg/analyzer_plugin/doc/tutorial/highlights.md
index 0296cafef5207563df11c6c828637cb95f10908e..23c6b7fd427205b87e0fdf4cdc84b43823713bb7 100644
--- a/pkg/analyzer_plugin/doc/tutorial/highlights.md
+++ b/pkg/analyzer_plugin/doc/tutorial/highlights.md
@@ -68,7 +68,7 @@ class HighlightsVisitor extends RecursiveAstVisitor {
   HighlightsVisitor(this.collector);
 
   @override
-  void visitSimpleIdentifier(ClassDeclaration node) {
+  void visitSimpleIdentifier(SimpleIdentifier node) {
     // ...
   }
 }