Reorganize barback's source files.

pkg/barback/lib/src/cancelable_future.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 barback.cancelable_future;
-
-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, stackTrace) {
-      if (_canceled) return;
-      _completer.completeError(error, stackTrace);
-    });
-  }
-
-  Stream<T> asStream() => _completer.future.asStream();
-  Future catchError(Function onError, {bool test(error)}) =>
-    _completer.future.catchError(onError, test: test);
-  Future then(onValue(T value), {Function onError}) =>
-    _completer.future.then(onValue, onError: onError);
-  Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
-  Future timeout(Duration timeLimit, {void onTimeout()}) =>
-    _completer.future.timeout(timeLimit, onTimeout: onTimeout);
-  /// Cancels this future.
-  void cancel() {
-    _canceled = true;
-  }
-}