Switch from AsyncThunk to AsyncMemoizer.

lib/src/runner/runner_suite.dart

Index: lib/src/runner/runner_suite.dart
diff --git a/lib/src/runner/runner_suite.dart b/lib/src/runner/runner_suite.dart
index 589178ff7b3d3ebe6994ae76293e392b6b3a22b6..10064afe244a84c8fcda7b4e9f23c07c8307297a 100644
--- a/lib/src/runner/runner_suite.dart
+++ b/lib/src/runner/runner_suite.dart
@@ -6,12 +6,13 @@ 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
@@ -21,8 +22,8 @@ import '../utils.dart';
 /// 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;
@@ -52,7 +53,7 @@ class RunnerSuite extends Suite {
 
   /// 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();
     });
   }