Don't treat the primary input as a secondary input in barback.

pkg/barback/lib/src/transform.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.transform;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'asset.dart';
@@ skipping 39 matching lines @@
     return _node.primary.asset;
   }
 
   Transform._(TransformNode node)
     : _node = node,
       super(node);
 
   /// Gets the asset for an input [id].
   ///
   /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
-  Future<Asset> getInput(AssetId id) => _node.getInput(id);
+  Future<Asset> getInput(AssetId id) {
+    if (id == _node.primary.id) return syncFuture(() => primaryInput);
+    return _node.getInput(id);
+  }
 
   /// A convenience method to the contents of the input with [id] as a string.
   ///
   /// This is equivalent to calling [getInput] followed by [Asset.readAsString].
   ///
   /// If the asset was created from a [String] the original string is always
   /// returned and [encoding] is ignored. Otherwise, the binary data of the
   /// asset is decoded using [encoding], which defaults to [UTF8].
   ///
   /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
@@ skipping 36 matching lines @@
 /// The controller for [Transform].
 class TransformController extends BaseTransformController {
   Transform get transform => super.transform;
 
   /// The set of assets that the transformer has emitted.
   AssetSet get outputs => transform._outputs;
 
   TransformController(TransformNode node)
       : super(new Transform._(node));
 }