Drop support for older Barback versions.

lib/src/asset/dart/serialize/aggregate_transform.dart

Index: lib/src/asset/dart/serialize/aggregate_transform.dart
diff --git a/lib/src/asset/dart/serialize/aggregate_transform.dart b/lib/src/asset/dart/serialize/aggregate_transform.dart
index 5b7acc816d86ae2f7163dabe9ef38a6108ba8a95..8efa85fe8835dc911992d5ba7322c26015136131 100644
--- a/lib/src/asset/dart/serialize/aggregate_transform.dart
+++ b/lib/src/asset/dart/serialize/aggregate_transform.dart
@@ -120,7 +120,6 @@ class _ForeignBaseAggregateTransform {
 // barback versions because merely referencing undefined types in type
 // annotations isn't a static error. Only implementing an undefined interface is
 // a static error.
-//# if barback >=0.14.1
 
 /// A wrapper for an [AggregateTransform] that's in the host isolate.
 ///
@@ -169,5 +168,3 @@ class ForeignDeclaringAggregateTransform
     });
   }
 }
-
-//# end