Add more documentation for plugins

pkg/analyzer_plugin/doc/tutorial/completion.md

Index: pkg/analyzer_plugin/doc/tutorial/completion.md
diff --git a/pkg/analyzer_plugin/doc/tutorial/completion.md b/pkg/analyzer_plugin/doc/tutorial/completion.md
index 6c33d13e86d652c76b161831476656b7bad33b4e..883921c96e86aea214d439739a4f90d010ecbfd9 100644
--- a/pkg/analyzer_plugin/doc/tutorial/completion.md
+++ b/pkg/analyzer_plugin/doc/tutorial/completion.md
@@ -58,7 +58,7 @@ For example, your contributor might look something like the following:
 ```dart
 class MyCompletionContributor implements CompletionContributor {
   @override
-  Future<Null> computeSuggestions(covariant DartCompletionRequest request,
+  Future<Null> computeSuggestions(DartCompletionRequest request,
       CompletionCollector collector) async {
     // ...
   }
@@ -74,7 +74,7 @@ class MyPlugin extends ServerPlugin with CompletionMixin, DartCompletionMixin {
 
   @override
   List<CompletionContributor> getCompletionContributors(
-        covariant AnalysisDriverGeneric driver) {
+      AnalysisDriverGeneric driver) {
     return <CompletionContributor>[new MyCompletionContributor()];
   }
 }