Throw a more descriptive error if a TransformerGroup has null phases.

sdk/lib/_internal/pub/asset/dart/serialize/transformer.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.serialize.transformer;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
@@ skipping 77 matching lines @@
 
   return {
     'type': type,
     'toString': transformer.toString(),
     'port': port.sendPort
   };
 }
 
 // Converts [group] into a serializable map.
 Map _serializeTransformerGroup(TransformerGroup group) {
+  if (group.phases == null) {
+    throw "TransformerGroup $group phases cannot be null.";
+  }
+
   return {
     'type': 'TransformerGroup',
     'toString': group.toString(),
     'phases': group.phases.map((phase) {
       return phase.map(serializeTransformerLike).toList();
     }).toList()
   };
 }
 
 /// Converts [transformerLike] into a serializable map.
 ///
 /// [transformerLike] can be a [Transformer], an [AggregateTransformer], or a
 /// [TransformerGroup].
 Map serializeTransformerLike(transformerLike) {
   if (transformerLike is Transformer) {
     return _serializeTransformer(transformerLike);
   } else if (transformerLike is TransformerGroup) {
     return _serializeTransformerGroup(transformerLike);
   } else {
     // This has to be last, since "transformerLike is AggregateTransformer" will
     // throw on older versions of barback.
     assert(transformerLike is AggregateTransformer);
     return _serializeAggregateTransformer(transformerLike);
   }
 }