Support passing barback assets through phases in which they're unused.

pkg/barback/lib/src/errors.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.errors;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 11 matching lines @@
 }
 
 /// The interface for exceptions from the barback graph or its transformers.
 ///
 /// These exceptions are never produced by programming errors in barback.
 abstract class BarbackException implements Exception {}
 
 /// Error thrown when two or more transformers both output an asset with [id].
 class AssetCollisionException implements BarbackException {
   /// All the transforms that output an asset with [id].
+  ///
+  /// If this only contains a single transform, that indicates that a
+  /// transformer produced an output that collides with a source asset or an
+  /// asset from a previous phase.
   final Set<TransformInfo> transforms;
   final AssetId id;
 
   AssetCollisionException(Iterable<TransformInfo> transforms, this.id)
       : transforms = new Set.from(transforms);
 
   String toString() => "Transforms $transforms all emitted asset $id.";
 }
 
 /// Error thrown when a transformer requests an input [id] which cannot be
@@ skipping 68 matching lines @@
 
   bool operator==(other) =>
       other is TransformInfo &&
       other.transformer == transformer &&
       other.primaryId == primaryId;
 
   int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 
   String toString() => "$transformer on $primaryId";
 }