Only pass an AssetId to isPrimary and declareOutputs.

pkg/barback/lib/src/transform.dart

Index: pkg/barback/lib/src/transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/transform.dart
index 1695388d3d98839fa09a2f0bc534f68a707c2a29..25c260af53a809be0af5c067fb6af2f9d3de7a0a 100644
--- a/pkg/barback/lib/src/transform.dart
+++ b/pkg/barback/lib/src/transform.dart
@@ -4,10 +4,16 @@
 
 library barback.transform;
 
+import 'dart:async';
+import 'dart:convert';
+
 import 'asset.dart';
+import 'asset_id.dart';
 import 'asset_set.dart';
 import 'base_transform.dart';
+import 'errors.dart';
 import 'transform_node.dart';
+import 'utils.dart';
 
 /// While a [Transformer] represents a *kind* of transformation, this defines
 /// one specific usage of it on a set of files.
@@ -17,10 +23,76 @@ import 'transform_node.dart';
 /// the transformation. It lets the [Transformer] access inputs and generate
 /// outputs.
 class Transform extends BaseTransform {
+  final TransformNode _node;
+
   final _outputs = new AssetSet();
 
+  /// Gets the primary input asset.
+  ///
+  /// While a transformation can use multiple input assets, one must be a
+  /// special "primary" asset. This will be the "entrypoint" or "main" input
+  /// file for a transformation.
+  ///
+  /// For example, with a dart2js transform, the primary input would be the
+  /// entrypoint Dart file. All of the other Dart files that that imports
+  /// would be secondary inputs.
+  ///
+  /// This method may fail at runtime with an [AssetNotFoundException] if called
+  /// asynchronously after the transform begins running. The primary input may
+  /// become unavailable while this transformer is running due to asset changes
+  /// earlier in the graph. You can ignore the error if this happens: the
+  /// transformer will be re-run automatically for you.
+  Asset get primaryInput {
+    if (!_node.primary.state.isAvailable) {
+      throw new AssetNotFoundException(_node.primary.id);
+    }
+
+    return _node.primary.asset;
+  }
+
   Transform._(TransformNode node)
-    : super(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);
+
+  /// 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].
+  Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
+    if (encoding == null) encoding = UTF8;
+    return getInput(id).then((input) => input.readAsString(encoding: encoding));
+  }
+
+  /// 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;
+    });
+  }
 
   /// Stores [output] as the output created by this transformation.
   ///