Add file missed in previous CL and comment clean-up

pkg/analysis_server/lib/completion/completion_dart.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analysis_server.completion.completion_dart;
 
 import 'package:analysis_server/completion/completion_core.dart';
 import 'package:analyzer/src/generated/ast.dart';
 
+/**
+ * An object used to produce completions for a specific error within a Dart
+ * file. Completion contributors are long-lived objects and must not retain any
+ * state between invocations of [computeSuggestions].
+ *
+ * Clients are expected to subtype this class when implementing plugins.
+ */
 abstract class DartCompletionContributor extends CompletionContributor {
   @override
   CompletionResult computeSuggestions(CompletionRequest request) {
     // TODO(brianwilkerson) Implement this by getting the information required
     // to create a DartCompletionRequest and calling:
     // return internalComputeSuggestions(dartRequest);
     return null;
   }
 
   /**
@@ skipping 28 matching lines @@
    * The completion target.  This determines what part of the parse tree
    * will receive the newly inserted text.
    */
   //CompletionTarget get target;
 
   /**
    * Information about the types of suggestions that should be included.
    */
   //OpType get _optype;
 }