Make dart2js unittests async.

tests/compiler/dart2js/reexport_handled_test.dart

 // Copyright (c) 2013, 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 reexport_handled_test;
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart'
     show Element,
          LibraryElement;
 
 final exportingLibraryUri = Uri.parse('exporting.dart');
 const String EXPORTING_LIBRARY_SOURCE = '''
 library exporting;
 var foo;
 ''';
 
 final reexportingLibraryUri = Uri.parse('reexporting.dart');
 const String REEXPORTING_LIBRARY_SOURCE = '''
 library reexporting;
 export 'exporting.dart';
 ''';
 
 void main() {
-  var compiler = new MockCompiler();
-  compiler.registerSource(exportingLibraryUri, EXPORTING_LIBRARY_SOURCE);
-  compiler.registerSource(reexportingLibraryUri, REEXPORTING_LIBRARY_SOURCE);
-
-  // Load exporting library before the reexporting library.
-  asyncTest(() => compiler.libraryLoader.loadLibrary(
-      exportingLibraryUri, null, exportingLibraryUri).then((exportingLibrary) {
+  var compiler;
+  asyncTest(() => MockCompiler.create((MockCompiler c) {
+    compiler = c;
+    compiler.registerSource(exportingLibraryUri, EXPORTING_LIBRARY_SOURCE);
+    compiler.registerSource(reexportingLibraryUri, REEXPORTING_LIBRARY_SOURCE);
+    return compiler.libraryLoader.loadLibrary(
+        exportingLibraryUri, null, exportingLibraryUri);
+  }).then((exportingLibrary) {
     Expect.isTrue(exportingLibrary.exportsHandled);
     var foo = findInExports(exportingLibrary, 'foo');
     Expect.isNotNull(foo);
     Expect.isTrue(foo.isField);
 
     // Load reexporting library when exports are handled on the exporting library.
     return compiler.libraryLoader.loadLibrary(
         reexportingLibraryUri, null, reexportingLibraryUri);
   }).then((reexportingLibrary) {
     var foo = findInExports(reexportingLibrary, 'foo');
     Expect.isNotNull(foo);
     Expect.isTrue(foo.isField);
   }));
 }
 
 Element findInExports(LibraryElement library, String name) {
   for (var export in library.exports) {
     if (export.name == name) {
       return export;
     }
   }
   return null;
 }