Refactor the barback tests somewhat.

pkg/barback/lib/src/cancelable_future.dart

+import 'dart:async';
+
+/// A wrapper for [Future] that can be cancelled.
+///
+/// When this is cancelled, that means it won't complete either successfully or
+/// with an error, regardless of whether the wrapped Future completes.
+/// Cancelling this won't stop whatever code is feeding the wrapped future from
+/// running.
+class CancelableFuture<T> implements Future<T> {
+  bool _canceled = false;
+  final _completer = new Completer<T>();
+
+  CancelableFuture(Future<T> inner) {
+    inner.then((result) {
+      if (_canceled) return;
+      _completer.complete(result);
+    }).catchError((error) {
+      if (_canceled) return;
+      _completer.completeError(error);
+    });
+  }
+
+  Stream<T> asStream() => _completer.future.asStream();
+  Future catchError(onError(asyncError), {bool test(error)}) =>
+    _completer.future.catchError(onError, test: test);
+  Future then(onValue(T value), {onError(error)}) =>
+    _completer.future.then(onValue, onError: onError);
+  Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
+
+  /// Cancels this future.
+  void cancel() {
+    _canceled = true;
+  }
+}