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

sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart

Index: sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart
diff --git a/sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart b/sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart
index 0e4d0c491b0552bdb4bfc26e54bb1bfdc72fdd83..16d02deb5790116cc24428521f357ba1fbd6f872 100644
--- a/sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart
+++ b/sdk/lib/_internal/pub/asset/dart/serialize/transformer.dart
@@ -95,6 +95,10 @@ Map _serializeAggregateTransformer(AggregateTransformer transformer) {
 
 // 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(),