Add a [Transform.hasInput] convenience method to Barback.

pkg/barback/lib/src/base_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.base_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'asset.dart';
@@ skipping 92 matching lines @@
   /// A convenience method to the contents of the input with [id].
   ///
   /// This is equivalent to calling [getInput] followed by [Asset.read].
   ///
   /// If the asset was created from a [String], this returns its UTF-8 encoding.
   ///
   /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
   Stream<List<int>> readInput(AssetId id) =>
       futureStream(getInput(id).then((input) => input.read()));
 
+  /// A convenience method to return whether or not an asset exists.
+  ///
+  /// This is equivalent to calling [getInput] and catching an
+  /// [AssetNotFoundException].
+  Future<bool> hasInput(AssetId id) {
+    return getInput(id).then((_) => true).catchError((error) {
+      if (error is AssetNotFoundException && error.id == id) return false;
+      throw error;
+    });
+  }
+
   /// Consume the primary input so that it doesn't get processed by future
   /// phases or emitted once processing has finished.
   ///
   /// Normally the primary input will automatically be forwarded unless the
   /// transformer overwrites it by emitting an input with the same id. This
   /// allows the transformer to tell barback not to forward the primary input
   /// even if it's not overwritten.
   void consumePrimary() {
     _consumePrimary = true;
   }
@@ skipping 20 matching lines @@
 
   /// Notifies the [BaseTransform] that the transformation has finished being
   /// applied.
   ///
   /// This will close any streams and release any resources that were allocated
   /// for the duration of the transformation.
   void close() {
     transform._onLogController.close();
   }
 }