Add explicit generics to Future.sync calls

lib/src/async_memoizer.dart

@@ -40,7 +40,7 @@ class AsyncMemoizer<T> {
   ///
   /// If [runOnce] has already been called, this returns the original result.
   Future<T> runOnce(FutureOr<T> Function() computation) {
-    if (!hasRun) _completer.complete(Future.sync(computation));
+    if (!hasRun) _completer.complete(Future<T>.sync(computation));
     return future;
   }
 }

lib/src/cancelable_operation.dart

@@ -105,17 +105,17 @@ class CancelableOperation<T> {
     valueOrCancellation().then((T result) {
       if (!completer.isCanceled) {
         if (isCompleted) {
-          completer.complete(Future.sync(() => onValue(result)));
+          completer.complete(Future<R>.sync(() => onValue(result)));
         } else if (onCancel != null) {
-          completer.complete(Future.sync(onCancel));
+          completer.complete(Future<R>.sync(onCancel));
         } else {
           completer._cancel();
         }
       }
     }, onError: (Object error, StackTrace stackTrace) {
       if (!completer.isCanceled) {
         if (onError != null) {
-          completer.complete(Future.sync(() => onError(error, stackTrace)));
+          completer.complete(Future<R>.sync(() => onError(error, stackTrace)));
         } else {
           completer.completeError(error, stackTrace);
         }