Switch from AsyncThunk to AsyncMemoizer.

lib/src/runner/runner_suite.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.runner.runner_suite;
 
 import 'dart:async';
 
+import 'package:async/async.dart';
+
 import '../backend/metadata.dart';
 import '../backend/operating_system.dart';
 import '../backend/suite.dart';
 import '../backend/test.dart';
 import '../backend/test_platform.dart';
-import '../util/async_thunk.dart';
 import '../utils.dart';
 
 /// A suite produced and consumed by the test runner that has runner-specific
 /// logic and lifecycle management.
 ///
 /// This is separated from [Suite] because the backend library (which will
 /// eventually become its own package) is primarily for test code itself to use,
 /// for which the [RunnerSuite] APIs don't make sense.
 class RunnerSuite extends Suite {
-  /// The thunk for running [close] exactly once.
-  final _closeThunk = new AsyncThunk();
+  /// The memoizer for running [close] exactly once.
+  final _closeMemo = new AsyncMemoizer();
 
   /// The function to call when the suite is closed.
   final AsyncFunction _onClose;
 
   RunnerSuite(Iterable<Test> tests, {String path, TestPlatform platform,
           OperatingSystem os, Metadata metadata, AsyncFunction onClose})
       : super(tests,
           path: path, platform: platform, os: os, metadata: metadata),
         _onClose = onClose;
 
   /// Creates a new [RunnerSuite] with the same properties as [suite].
   RunnerSuite.fromSuite(Suite suite)
       : super(suite.tests,
           path: suite.path,
           platform: suite.platform,
           os: suite.os,
           metadata: suite.metadata),
         _onClose = null;
 
   RunnerSuite change({String path, Metadata metadata, Iterable<Test> tests}) {
     if (path == null) path = this.path;
     if (metadata == null) metadata = this.metadata;
     if (tests == null) tests = this.tests;
     return new RunnerSuite(tests, platform: platform, os: os, path: path,
         metadata: metadata, onClose: this.close);
   }
 
   /// Closes the suite and releases any resources associated with it.
   Future close() {
-    return _closeThunk.run(() async {
+    return _closeMemo.runOnce(() async {
       if (_onClose != null) await _onClose();
     });
   }
 }