Drop support for older Barback versions.

lib/src/asset/dart/transformer_isolate.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 pub.asset.transformer_isolate;
 
 import 'dart:convert';
 import 'dart:isolate';
 import 'dart:mirrors';
 
@@ skipping 21 matching lines @@
     });
   });
 }
 
 /// Loads all the transformers and groups defined in [uri].
 ///
 /// Loads the library, finds any [Transformer] or [TransformerGroup] subclasses
 /// in it, instantiates them with [configuration] and [mode], and returns them.
 List _initialize(String uri, Map configuration, BarbackMode mode) {
   var transformerClass = reflectClass(Transformer);
-  var aggregateClass = _aggregateTransformerClass;
+  var aggregateClass = reflectClass(AggregateTransformer);
   var groupClass = reflectClass(TransformerGroup);
 
   var seen = new Set();
   var transformers = [];
 
   loadFromLibrary(library) {
     if (seen.contains(library)) return;
     seen.add(library);
 
     // Load transformers from libraries exported by [library].
     for (var dependency in library.libraryDependencies) {
       if (!dependency.isExport) continue;
       loadFromLibrary(dependency.targetLibrary);
     }
 
     // TODO(nweiz): if no valid transformers are found, throw an error message
     // describing candidates and why they were rejected.
     transformers.addAll(library.declarations.values.map((declaration) {
       if (declaration is! ClassMirror) return null;
       var classMirror = declaration;
       if (classMirror.isPrivate) return null;
       if (classMirror.isAbstract) return null;
       if (!classMirror.isSubtypeOf(transformerClass) &&
           !classMirror.isSubtypeOf(groupClass) &&
-          (aggregateClass == null ||
-              !classMirror.isSubtypeOf(aggregateClass))) {
+          !classMirror.isSubtypeOf(aggregateClass)) {
         return null;
       }
 
       var constructor = _getConstructor(classMirror, 'asPlugin');
       if (constructor == null) return null;
       if (constructor.parameters.isEmpty) {
         if (configuration.isNotEmpty) return null;
         return classMirror.newInstance(const Symbol('asPlugin'), []).reflectee;
       }
       if (constructor.parameters.length != 1) return null;
@@ skipping 14 matching lines @@
 }
 
 // TODO(nweiz): clean this up when issue 13248 is fixed.
 MethodMirror _getConstructor(ClassMirror classMirror, String constructor) {
   var name = new Symbol("${MirrorSystem.getName(classMirror.simpleName)}"
       ".$constructor");
   var candidate = classMirror.declarations[name];
   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
-ClassMirror get _aggregateTransformerClass =>
-    reflectClass(AggregateTransformer);
-//# else
-//>   ClassMirror get _aggregateTransformerClass => null;
-//# end