Migrate async tests to strong

pkg/dev_compiler/test/codegen_test.dart

 // Copyright (c) 2016, 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.
 
 /// Tests code generation.
 ///
 /// Runs Dart Dev Compiler on all input in the `codegen` directory and checks
 /// that the output is what we expected.
 library dev_compiler.test.codegen_test;
 
@@ skipping 68 matching lines @@
       .where((p) => p.endsWith('.sum'))
       .toList();
 
   var sharedCompiler = new ModuleCompiler(new AnalyzerOptions.basic(
       dartSdkSummaryPath: sdkSummaryFile, summaryPaths: summaryPaths));
 
   var testDirs = [
     'language',
     'corelib',
     path.join('corelib', 'regexp'),
+    path.join('lib', 'async'),
     path.join('lib', 'collection'),
     path.join('lib', 'convert'),
     path.join('lib', 'html'),
     path.join('lib', 'math'),
     path.join('lib', 'mirrors'),
     path.join('lib', 'typed_data'),
   ];
 
   // Copy all of the test files and expanded multitest files to
   // gen/codegen_tests. We'll compile from there.
@@ skipping 295 matching lines @@
 
   // TODO(vsm): Fix these - they import files from a different directory
   // - this triggers an invalid library root build error.
   'lib/html/custom/attribute_changed_callback_test',
   'lib/html/custom/constructor_calls_created_synchronously_test',
   'lib/html/custom/entered_left_view_test',
   'lib/html/custom/js_custom_test',
   'lib/html/custom/mirrors_test',
   'lib/html/custom/regress_194523002_test',
 ]);