refactor code completion to allow for multiple suggestion computers

pkg/analysis_services/lib/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_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 {
-  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();
-    });
-  }
-}