refactor code completion to allow for multiple suggestion computers

pkg/analysis_services/lib/src/completion/top_level_computer.dart

 // Copyright (c) 2014, 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 services.completion.computer.toplevel;
 
 import 'dart:async';
 
+import 'package:analysis_services/completion/completion_computer.dart';
 import 'package:analysis_services/completion/completion_suggestion.dart';
 import 'package:analysis_services/search/search_engine.dart';
 import 'package:analyzer/src/generated/element.dart';
 
 /**
  * A computer for `completion.getSuggestions` request results.
  */
-class TopLevelComputer {
+class TopLevelComputer extends CompletionComputer {
   final SearchEngine searchEngine;
 
   TopLevelComputer(this.searchEngine);
 
   /**
    * Computes [CompletionSuggestion]s for the specified position in the source.
    */
   Future<List<CompletionSuggestion>> compute() {
     var future = searchEngine.searchTopLevelDeclarations('');
     return future.then((List<SearchMatch> matches) {
       return matches.map((SearchMatch match) {
         Element element = match.element;
         String completion = element.displayName;
         return new CompletionSuggestion(
             CompletionSuggestionKind.fromElementKind(element.kind),
             CompletionRelevance.DEFAULT,
             completion,
             completion.length,
             0,
             element.isDeprecated,
             false // isPotential
             );
       }).toList();
     });
   }
 }