Make subclasses of AbstractContextTest asynchronous.

pkg/analysis_server/test/services/dependencies/reachable_source_collector_test.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 test.services.dependencies.import_collector;
 
 import 'package:analysis_server/src/services/dependencies/reachable_source_collector.dart';
-import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../../abstract_context.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ReachableSourceCollectorTest);
   });
 }
 
 @reflectiveTest
 class ReachableSourceCollectorTest extends AbstractContextTest {
-  CompilationUnit addLibrary(String path, String contents) =>
-      resolveLibraryUnit(addSource(path, contents));
-
   Map<String, List<String>> importsFor(Source source) =>
       new ReachableSourceCollector(source, context).collectSources();
 
   test_null_context() {
     Source lib = addSource('/lib.dart', '');
     expect(() => new ReachableSourceCollector(lib, null),
         throwsA(new isInstanceOf<ArgumentError>()));
   }
 
   test_null_source() {
@@ skipping 30 matching lines @@
     // Cycles should be OK.
     {
       Map<String, List<String>> imports = importsFor(lib3);
       expect(imports.keys, contains('file:///lib3.dart'));
       expect(imports.keys, contains('file:///lib4.dart'));
       expect(imports.keys, contains('dart:core'));
       expect(imports.keys, contains('dart:math'));
     }
   }
 }