Stop working around issue 12826 in pub.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index c92791b6c67d20908336b8b280f449dde1c044f2..c5ab37fd7a0c8c79678c200361190dcd8d66e602 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -66,7 +66,7 @@ Iterable initialize(Uri uri, Map configuration, BarbackMode mode) {
     if (declaration is! ClassMirror) return null;
     var classMirror = declaration;
     if (classMirror.isPrivate) return null;
-    if (isAbstract(classMirror)) return null;
+    if (classMirror.isAbstract) return null;
     if (!classIsA(classMirror, transformerClass) &&
         !classIsA(classMirror, groupClass)) {
       return null;
@@ -164,11 +164,6 @@ bool classIsA(ClassMirror mirror, ClassMirror superclass) {
       mirror.superinterfaces.any((int) => classIsA(int, superclass));
 }
 
-// TODO(nweiz): get rid of this when issue 12826 is fixed.
-/// Returns whether or not [mirror] is an abstract class.
-bool isAbstract(ClassMirror mirror) => mirror.declarations.values
-    .any((member) => member is MethodMirror && member.isAbstract);
-
 /// Converts [transformerOrGroup] into a serializable map.
 Map _serializeTransformerOrGroup(transformerOrGroup) {
   if (transformerOrGroup is Transformer) {