Don't require Transformer methods to return Futures.

sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart

 // Copyright (c) 2014, 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 pub.asset.serialize.transformer;
 
+import 'dart:async';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
 
 import '../serialize.dart';
+import '../utils.dart';
 import 'transform.dart';
 
 /// Converts [transformer] into a serializable map.
 Map _serializeTransformer(Transformer transformer) {
   var port = new ReceivePort();
   port.listen((wrappedMessage) {
     respond(wrappedMessage, (message) {
       if (message['type'] == 'isPrimary') {
         return transformer.isPrimary(deserializeId(message['id']));
       } else if (message['type'] == 'declareOutputs') {
-        return (transformer as DeclaringTransformer).declareOutputs(
-            new ForeignDeclaringTransform(message['transform']))
-            .then((_) => null);
+        return new Future.sync(() {
+          return (transformer as DeclaringTransformer).declareOutputs(
+              new ForeignDeclaringTransform(message['transform']));
+        }).then((_) => null);
       } else {
         assert(message['type'] == 'apply');
 
         // Make sure we return null so that if the transformer's [apply] returns
         // a non-serializable value it doesn't cause problems.
-        return transformer.apply(
-            new ForeignTransform(message['transform'])).then((_) => null);
+        return new Future.sync(() {
+          return transformer.apply(new ForeignTransform(message['transform']));
+        }).then((_) => null);
       }
     });
   });
 
   var type;
   if (transformer is LazyTransformer) {
     type = 'LazyTransformer';
   } else if (transformer is DeclaringTransformer) {
     type = 'DeclaringTransformer';
   } else {
@@ skipping 20 matching lines @@
 
 /// Converts [transformerOrGroup] into a serializable map.
 Map serializeTransformerOrGroup(transformerOrGroup) {
   if (transformerOrGroup is Transformer) {
     return _serializeTransformer(transformerOrGroup);
   } else {
     assert(transformerOrGroup is TransformerGroup);
     return _serializeTransformerGroup(transformerOrGroup);
   }
 }