Expose aggregate transformers in pub.

dart/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart

Index: dart/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
diff --git a/dart/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart b/dart/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
index 3b668417ef5321553086efd41ce71ef461d7c1d3..003c5f0f212e64ebdcbbc340384b84f3ddacfc7e 100644
--- a/dart/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
+++ b/dart/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
@@ -22,7 +22,7 @@ void loadTransformers(SendPort replyTo) {
       var configuration = JSON.decode(message['configuration']);
       var mode = new BarbackMode(message['mode']);
       return _initialize(library, configuration, mode).
-          map(serializeTransformerOrGroup).toList();
+          map(serializeTransformerLike).toList();
     });
   });
 }
@@ -34,6 +34,7 @@ void loadTransformers(SendPort replyTo) {
 List _initialize(Uri uri, Map configuration, BarbackMode mode) {
   var mirrors = currentMirrorSystem();
   var transformerClass = reflectClass(Transformer);
+  var aggregateClass = _aggregateTransformerClass;
   var groupClass = reflectClass(TransformerGroup);
 
   var seen = new Set();
@@ -57,7 +58,9 @@ List _initialize(Uri uri, Map configuration, BarbackMode mode) {
       if (classMirror.isPrivate) return null;
       if (classMirror.isAbstract) return null;
       if (!classMirror.isSubtypeOf(transformerClass) &&
-          !classMirror.isSubtypeOf(groupClass)) {
+          !classMirror.isSubtypeOf(groupClass) &&
+          (aggregateClass == null ||
+              !classMirror.isSubtypeOf(aggregateClass))) {
         return null;
       }
 
@@ -86,3 +89,13 @@ MethodMirror _getConstructor(ClassMirror classMirror, String constructor) {
   if (candidate is MethodMirror && candidate.isConstructor) return candidate;
   return null;
 }
+
+// Older barbacks don't support [AggregateTransformer], and calling
+// [reflectClass] on an undefined class will throw an error, so we just define a
+// null getter for them.
+//# if barback >=0.14.1-dev
+ClassMirror get _aggregateTransformerClass =>
+    reflectClass(AggregateTransformer);
+//# else
+//>   ClassMirror get _aggregateTransformerClass => null;
+//# end