Add BaseTransform.consumePrimary to 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 37 matching lines @@
 
   /// Whether [this] is dirty and still has more processing to do.
   bool get isDirty => _pendingIsPrimary || _isApplying;
 
   /// Whether any input has become dirty since [_apply] last started running.
   var _hasBecomeDirty = false;
 
   /// Whether [_apply] is currently running.
   var _isApplying = false;
 
+  /// Whether the most recent run of this transform has declared that it
+  /// consumes the primary input.
+  ///
+  /// Defaults to `false`.
+  bool get consumePrimary => _consumePrimary;
+  bool _consumePrimary = false;
+
   /// Whether [transformer] is lazy and this transform has yet to be forced.
   bool _isLazy;
 
   /// The subscriptions to each input's [AssetNode.onStateChange] stream.
   var _inputSubscriptions = new Map<AssetId, StreamSubscription>();
 
   /// The controllers for the asset nodes emitted by this node.
   var _outputControllers = new Map<AssetId, AssetNodeController>();
 
   // TODO(nweiz): It's weird that this is different than the [onDone] stream the
@@ skipping 171 matching lines @@
   /// outputs.
   Future _applyImmediate() {
     var transformController = new TransformController(this);
     _onLogPool.add(transformController.onLog);
 
     return syncFuture(() {
       return transformer.apply(transformController.transform);
     }).then((_) {
       if (_hasBecomeDirty || _onAssetController.isClosed) return;
 
+      _consumePrimary = transformController.consumePrimary;
+
       var newOutputs = transformController.outputs;
       // Any ids that are for a different package are invalid.
       var invalidIds = newOutputs
           .map((asset) => asset.id)
           .where((id) => id.package != phase.cascade.package)
           .toSet();
       for (var id in invalidIds) {
         newOutputs.removeId(id);
         // TODO(nweiz): report this as a warning rather than a failing error.
         phase.cascade.reportError(new InvalidOutputException(info, id));
@@ skipping 23 matching lines @@
   /// outputs.
   Future _declareLazy() {
     var transformController = new DeclaringTransformController(this);
 
     return syncFuture(() {
       return (transformer as LazyTransformer)
           .declareOutputs(transformController.transform);
     }).then((_) {
       if (_hasBecomeDirty || _onAssetController.isClosed) return;
 
+      _consumePrimary = transformController.consumePrimary;
+
       var newIds = transformController.outputIds;
       var invalidIds =
           newIds.where((id) => id.package != phase.cascade.package).toSet();
       for (var id in invalidIds) {
         newIds.remove(id);
         // TODO(nweiz): report this as a warning rather than a failing error.
         phase.cascade.reportError(new InvalidOutputException(info, id));
       }
 
       // Remove outputs that used to exist but don't anymore.
@@ skipping 11 matching lines @@
           _outputControllers[id] = controller;
           _onAssetController.add(controller.node);
         }
       }
     });
   }
 
   String toString() =>
     "transform node in $_location for $transformer on $primary";
 }