Fix all strong-mode warnings.

lib/src/graph/transformer_classifier.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.graph.transformer_classifier;
 
 import 'dart:async';
 
 import '../asset/asset_forwarder.dart';
 import '../asset/asset_node.dart';
 import '../errors.dart';
 import '../log.dart';
 import '../transformer/aggregate_transformer.dart';
 import '../transformer/wrapping_aggregate_transformer.dart';
-import '../utils.dart';
 import 'node_status.dart';
 import 'node_streams.dart';
 import 'phase.dart';
 import 'transform_node.dart';
 
 /// A class for classifying the primary inputs for a transformer according to
 /// its [AggregateTransformer.classifyPrimary] method.
 ///
 /// This is also used for non-aggregate transformers; they're modeled as
 /// aggregate transformers that return the primary path if `isPrimary` is true
@@ skipping 41 matching lines @@
         _transforms.values.map((transform) => transform.status));
   }
 
   TransformerClassifier(this.phase, transformer, this._location)
       : transformer = transformer is AggregateTransformer ?
             transformer : new WrappingAggregateTransformer(transformer);
 
   /// Adds a new asset as an input for this transformer.
   void addInput(AssetNode input) {
     _activeClassifications++;
-    syncFuture(() => transformer.classifyPrimary(input.id)).catchError(
-        (error, stackTrace) {
+    new Future.sync(() => transformer.classifyPrimary(input.id))
+        .catchError((error, stackTrace) {
       if (input.state.isRemoved) return null;
 
       // Catch all transformer errors and pipe them to the results stream. This
       // is so a broken transformer doesn't take down the whole graph.
       var info = new TransformInfo(transformer, input.id);
       if (error is! AssetNotFoundException) {
         error = new TransformerException(info, error, stackTrace);
       } else {
         error = new MissingInputException(info, error.id);
       }
@@ skipping 49 matching lines @@
 
   /// Force all deferred transforms to begin producing concrete assets.
   void forceAllTransforms() {
     for (var transform in _transforms.values) {
       transform.force();
     }
   }
 
   String toString() => "classifier in $_location for $transformer";
 }