Add more documentation for plugins

pkg/analyzer_plugin/doc/tutorial/completion.md

 # Providing Code Completions
 
 A code completion is used by clients to provide a set of possible completions to
 partially entered code. Completions are intended to address two use cases: to
 help users enter code with less effort and to help users discover the behavior
 of an object.
 
 For example, if the user has typed `o.toSt` and then requested completions, one
 suggestion might be `toString`.
 
@@ skipping 40 matching lines @@
 implement the method `computeSuggestions`. Your contributor should invoke the
 method `checkAborted`, defined on the `CompletionRequest` object, before
 starting any slow work. This allows the computation of completion suggestions
 to be preempted if the client no longer needs the results.
 
 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 {
     // ...
   }
 }
 ```
 
 Given a contributor like the one above, you can implement your plugin similar to
 the following:
 
 ```dart
 class MyPlugin extends ServerPlugin with CompletionMixin, DartCompletionMixin {
   // ...
 
   @override
   List<CompletionContributor> getCompletionContributors(
-        covariant AnalysisDriverGeneric driver) {
+      AnalysisDriverGeneric driver) {
     return <CompletionContributor>[new MyCompletionContributor()];
   }
 }
 ```