Asset load failures in Barback should produce AssetNotFoundExceptions.

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 34 matching lines @@
   /// 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 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.
+  /// 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 != AssetState.AVAILABLE) {
       throw new AssetNotFoundException(_node.primary.id);
     }
 
     return _node.primary.asset;
   }
 
   BaseTransform(this._node) {
     _logger = new TransformLogger((asset, level, message, span) {
       // If the log isn't already associated with an asset, use the primary.
       if (asset == null) asset = _node.primary.id;
       var entry = new LogEntry(_node.info, asset, level, message, span);
       _onLogController.add(entry);
     });
   }
 
   /// Gets the asset for an input [id].
   ///
-  /// If an input with that ID cannot be found, throws an
-  /// [AssetNotFoundException].
+  /// 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 `readAsString()`.
+  /// 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 `read()`.
+  /// 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()));
 
   /// 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.
@@ 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();
   }
 }