Fix analyzer errors in barback.

pkg/barback/lib/src/transform_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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 202 matching lines @@
 
   /// Applies this transform.
   void _apply() {
     assert(!_onAssetController.isClosed);
 
     // Clear input subscriptions here as well as in [_process] because [_apply]
     // may be restarted independently if only a secondary input changes.
     _clearInputSubscriptions();
     _state = _TransformNodeState.PROCESSING;
     primary.whenAvailable((_) {
-      if (_state.needsIsPrimary) return;
+      if (_state.needsIsPrimary) return null;
       _state = _TransformNodeState.PROCESSING;
       // TODO(nweiz): If [transformer] is a [DeclaringTransformer] but not a
       // [LazyTransformer], we can get some mileage out of doing a declarative
       // first so we know how to hook up the assets.
       if (_isLazy) return _declareLazy();
       return _applyImmediate();
     }).catchError((error, stackTrace) {
       // If the transform became dirty while processing, ignore any errors from
       // it.
       if (!_state.isProcessing || _isRemoved) return;
@@ skipping 249 matching lines @@
   ///
   /// Specifically, whether [this] is [APPLIED] or [NOT_PRIMARY].
   bool get isDone => isApplied || isNotPrimary;
 
   final String name;
 
   const _TransformNodeState._(this.name);
 
   String toString() => name;
 }