Don't require Transformer methods to return Futures.

pkg/barback/lib/src/transformer.dart

Index: pkg/barback/lib/src/transformer.dart
diff --git a/pkg/barback/lib/src/transformer.dart b/pkg/barback/lib/src/transformer.dart
index 2b9499af2b8fa85bdd97f75f93d0eda202598384..acac1fda8ff872410222ead77a0886c3b7903877 100644
--- a/pkg/barback/lib/src/transformer.dart
+++ b/pkg/barback/lib/src/transformer.dart
@@ -59,15 +59,17 @@ abstract class Transformer {
   /// 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 either a `bool` or a `Future<bool>`.

[Bob Nystrom, 2014/04/21 22:32:23]

Here too. Make it clear that not returning a future is only valid if the
implementation does nothing async.

[nweiz, 2014/04/21 22:46:24]

Done.

+  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].
@@ -81,7 +83,10 @@ abstract class Transformer {
   /// 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);
+  ///
+  /// This may return a [Future]. If it does, it won't be considered finished
+  /// until that [Future] completes.

[Bob Nystrom, 2014/04/21 22:32:23]

Ditto.

[nweiz, 2014/04/21 22:46:24]

Done.

+  apply(Transform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }