Add an Invoker class that manages a running test.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index ea1e92ea7d939bcd7be3812ad1e1fe8371fb9977..526724128f9831a090650af448acdc7a557ac8ef 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -4,8 +4,15 @@
 
 library unittest.utils;
 
+import 'dart:async';
+
 import 'package:stack_trace/stack_trace.dart';
 
+/// A typedef for a possibly-asynchronous function.
+///
+/// The return type should only ever by [Future] or void.
+typedef AsyncFunction();
+
 /// Indent each line in [str] by two spaces.
 String indent(String str) =>
     str.replaceAll(new RegExp("^", multiLine: true), "  ");