Fix most strong mode warnings.

lib/src/cancelable_operation.dart

Index: lib/src/cancelable_operation.dart
diff --git a/lib/src/cancelable_operation.dart b/lib/src/cancelable_operation.dart
index 1ba6b3c4759f5b8477fb8740c51ad9c0dfb8cde5..ab8940b911ea0b6b4c39b375720e9188215c4fd5 100644
--- a/lib/src/cancelable_operation.dart
+++ b/lib/src/cancelable_operation.dart
@@ -62,9 +62,8 @@ class CancelableOperation<T> {
   /// 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);