Fix all strong-mode warnings.

test/transformer/aggregate_many_to_one.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.aggregate_many_to_one;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
@@ skipping 14 matching lines @@
   /// well.
   final String output;
 
   AggregateManyToOneTransformer(this.extension, this.output);
 
   String doClassifyPrimary(AssetId id) {
     if (id.extension != ".$extension") return null;
     return path.url.dirname(id.path);
   }
 
-  Future doApply(AggregateTransform transform) {
-    return getPrimaryInputs(transform).toList().then((assets) {
-      assets.sort((asset1, asset2) => asset1.id.path.compareTo(asset2.id.path));
-      return Future.wait(assets.map((asset) => asset.readAsString()));
-    }).then((contents) {
-      var id = new AssetId(transform.package,
-          path.url.join(transform.key, output));
-      transform.addOutput(new Asset.fromString(id, contents.join('\n')));
-    });
+  Future doApply(AggregateTransform transform) async {
+    var assets = await getPrimaryInputs(transform).toList();
+    assets.sort((asset1, asset2) => asset1.id.path.compareTo(asset2.id.path));
+    var contents = await Future.wait(
+        assets.map((asset) => asset.readAsString()));
+    var id = new AssetId(transform.package,
+        path.url.join(transform.key, output));
+    transform.addOutput(new Asset.fromString(id, contents.join('\n')));
   }
 
   String toString() => "aggregate $extension->$output";
 }