Don't require Transformer methods to return Futures.

pkg/barback/lib/src/transformer.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.transformer;
 
 import 'dart:async';
 
 import 'asset_id.dart';
 import 'transform.dart';
@@ skipping 41 matching lines @@
   ///
   /// A concrete example is dart2js. When you run dart2js, it will traverse
   /// all of the imports in your Dart source files and use the contents of all
   /// of those to generate the final JS. However you still run dart2js "on" a
   /// single file: the entrypoint Dart file that has your `main()` method.
   /// This entrypoint file would be the primary input.
   ///
   /// If this is not overridden, defaults to allow any asset whose extension
   /// matches one of the ones returned by [allowedExtensions]. If *that* is
   /// not overridden, allows all assets.
-  Future<bool> isPrimary(AssetId id) {
+  ///
+  /// This may return a `Future<bool>` or, if it's entirely synchronous, a
+  /// `bool`.
+  isPrimary(AssetId id) {
     // Allow all files if [primaryExtensions] is not overridden.
-    if (allowedExtensions == null) return new Future.value(true);
+    if (allowedExtensions == null) return true;
 
     for (var extension in allowedExtensions.split(" ")) {
-      if (id.path.endsWith(extension)) return new Future.value(true);
+      if (id.path.endsWith(extension)) return true;
     }
 
-    return new Future.value(false);
+    return false;
   }
 
   /// Run this transformer on on the primary input specified by [transform].
   ///
   /// The [transform] is used by the [Transformer] for two purposes (in
   /// addition to accessing the primary input). It can call `getInput()` to
   /// request additional input assets. It also calls `addOutput()` to provide
   /// generated assets back to the system. Either can be called multiple times,
   /// in any order.
   ///
   /// In other words, a Transformer's job is to find all inputs for a
   /// transform, starting at the primary input, then generate all output assets
   /// and yield them back to the transform.
-  Future apply(Transform transform);
+  ///
+  /// If this does asynchronous work, it should return a [Future] that completes
+  /// once it's finished.
+  apply(Transform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }