Fix all strong-mode warnings.

test/transformer/sync_rewrite.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 barback.test.transformer.sync_rewrite;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 
 /// Like [DeclaringRewriteTransformer], but with no methods returning Futures.
 class SyncRewriteTransformer extends Transformer
     implements DeclaringTransformer {
   final String from;
   final String to;
 
   SyncRewriteTransformer(this.from, this.to);
 
   bool isPrimary(AssetId id) => id.extension == ".$from";
 
   void apply(Transform transform) {
     for (var extension in to.split(" ")) {
       var id = transform.primaryInput.id.changeExtension(".$extension");
       transform.addOutput(new Asset.fromString(id, "new.$extension"));
     }
   }
 
   void declareOutputs(DeclaringTransform transform) {
     for (var extension in to.split(" ")) {
       var id = transform.primaryId.changeExtension(".$extension");
       transform.declareOutput(id);
     }
   }
 
   String toString() => "$from->$to";
 }