refactor code completion to allow for multiple suggestion computers

pkg/analysis_services/test/completion/test_all.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.services.completion;
 
 import 'package:unittest/unittest.dart';
 
-import 'top_level_computer_test.dart' as toplevel_test;
+import 'completion_computer_test.dart' as completion_test;
+import 'top_level_computer_test.dart' as topLevel_test;
 
 /// Utility for manually running all tests.
 main() {
   groupSep = ' | ';
   group('completion', () {
-    toplevel_test.main();
+    completion_test.main();
+    topLevel_test.main();
   });
 }