Fix most strong mode warnings.

lib/src/cancelable_operation.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 
 import 'package:async/async.dart';
 
 /// An asynchronous operation that can be cancelled.
 ///
@@ skipping 44 matching lines @@
     return controller.stream;
   }
 
   /// Creates a [Future] that completes when this operation completes *or* when
   /// it's cancelled.
   ///
   /// If this operation completes, this completes to the same result as [value].
   /// If this operation is cancelled, the returned future waits for the future
   /// returned by [cancel], then completes to [cancellationValue].
   Future valueOrCancellation([T cancellationValue]) {
-    var completer = new Completer.sync();
-
-    value.then(completer.complete, onError: completer.completeError);
+    var completer = new Completer<T>.sync();
+    completer.complete(value);
 
     _completer._cancelMemo.future.then((_) {
       completer.complete(cancellationValue);
     }, onError: completer.completeError);
 
     return completer.future;
   }
 
   /// Cancels this operation.
   ///
@@ skipping 80 matching lines @@
   /// Cancel the completer.
   Future _cancel() {
     if (_inner.isCompleted) return new Future.value();
 
     return _cancelMemo.runOnce(() {
       _isCanceled = true;
       if (_onCancel != null) return _onCancel();
     });
   }
 }