refactor code completion to allow for multiple suggestion computers

pkg/analysis_server/test/domain_completion_test.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 test.domain.completion;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/domain_completion.dart';
@@ skipping 120 matching lines @@
       var test^ = '';
       main() {test.}
     ''');
     return getSuggestions().then((_) {
       assertHasResult(CompletionSuggestionKind.CLASS, 'Object');
       assertHasResult(CompletionSuggestionKind.TOP_LEVEL_VARIABLE, 'test');
       assertNoResult('HtmlElement');
     });
   }
 }