Fix all strong-mode warnings.

lib/src/asset/asset_node.dart

 // Copyright (c) 2013, 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.asset.asset_node;
 
 import 'dart:async';
 
 import '../errors.dart';
 import '../graph/transform_node.dart';
-import '../utils.dart';
 import 'asset.dart';
 import 'asset_id.dart';
 
 /// Describes the current state of an asset as part of a transformation graph.
 ///
 /// An asset node can be in one of three states (see [AssetState]). It provides
 /// an [onStateChange] stream that emits an event whenever it changes state.
 ///
 /// Asset nodes are controlled using [AssetNodeController]s.
 class AssetNode {
@@ skipping 86 matching lines @@
   Future<AssetState> whenStateChanges() {
     var startState = state;
     return _waitForState((state) => state != startState, (state) => state);
   }
 
   /// Calls [callback] as soon as the node is in a state that matches [test].
   ///
   /// [callback] is called synchronously if this is already in such a state.
   ///
   /// The return value of [callback] is piped to the returned Future.
-  Future _waitForState(bool test(AssetState state),
-      callback(AssetState state)) {
-    if (test(state)) return syncFuture(() => callback(state));
+  Future/*<T>*/ _waitForState/*<T>*/(bool test(AssetState state),
+      /*=T*/ callback(AssetState state)) {
+    if (test(state)) return new Future.sync(() => callback(state));
     return onStateChange.firstWhere(test).then((_) => callback(state));
   }
 
   AssetNode._(this.id, this._transform, this._origin)
       : _state = AssetState.RUNNING;
 
   AssetNode._available(Asset asset, this._transform, this._origin)
       : id = asset.id,
         _asset = asset,
         _state = AssetState.AVAILABLE;
@@ skipping 143 matching lines @@
 
   /// Whether this state is [AssetState.RUNNING].
   bool get isDirty => this == AssetState.RUNNING;
 
   final String name;
 
   const AssetState._(this.name);
 
   String toString() => name;
 }