Use the async package's CancelableOperation.

lib/src/runner/environment.dart

Index: lib/src/runner/environment.dart
diff --git a/lib/src/runner/environment.dart b/lib/src/runner/environment.dart
index 33911c4cb9d43cffd3445edbce43711dc25de3a2..f490e0a770ecd61b1a10911eed55b1aec5230e58 100644
--- a/lib/src/runner/environment.dart
+++ b/lib/src/runner/environment.dart
@@ -4,7 +4,7 @@
 
 library test.runner.environment;
 
-import '../util/cancelable_future.dart';
+import 'package:async/async.dart';
 
 /// The abstract class of environments in which test suites are
 /// loaded—specifically, browsers and the Dart VM.
@@ -19,8 +19,8 @@ abstract class Environment {
 
   /// Displays information indicating that the test runner is paused.
   ///
-  /// The returned future will complete when the user takes action within the
+  /// The returned operation will complete when the user takes action within the
   /// environment that should unpause the runner. If the runner is unpaused
-  /// elsewhere, the future should be canceled.
-  CancelableFuture displayPause();
+  /// elsewhere, the operation should be canceled.
+  CancelableOperation displayPause();
 }