Support configuring dart2js from the pubspec.

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 c5ab37fd7a0c8c79678c200361190dcd8d66e602..66938434704a247a78e057e5dd30ef11c67c534f 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -75,15 +75,11 @@ Iterable initialize(Uri uri, Map configuration, BarbackMode mode) {
     var constructor = getConstructor(classMirror, 'asPlugin');
     if (constructor == null) return null;
     if (constructor.parameters.isEmpty) {
-      if (configuration != null) return null;
+      if (configuration.isNotEmpty) return null;
       return classMirror.newInstance(const Symbol('asPlugin'), []).reflectee;
     }
     if (constructor.parameters.length != 1) return null;
 
-    // If the constructor expects configuration and none was passed, it defaults
-    // to an empty map.
-    if (configuration == null) configuration = {};
-
     return classMirror.newInstance(const Symbol('asPlugin'),
         [new BarbackSettings(configuration, mode)]).reflectee;
   }).where((classMirror) => classMirror != null);