Add an Invoker class that manages a running test.

lib/src/utils.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 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), "  ");
 
 /// A pair of values.
 class Pair<E, F> {
   final E first;
   final F last;
 
   Pair(this.first, this.last);
@@ skipping 23 matching lines @@
 
   if (!filterStacks) return trace;
 
   // Format the stack trace by removing everything above TestCase._runTest,
   // which is usually going to be irrelevant. Also fold together unittest and
   // core library calls so only the function the user called is visible.
   return new Trace(trace.frames.takeWhile((frame) {
     return frame.package != 'unittest' || frame.member != 'TestCase._runTest';
   })).terse.foldFrames((frame) => frame.package == 'unittest' || frame.isCore);
 }