Make pub strong-mode clean.

lib/src/barback/foreign_transformer.dart

Index: lib/src/barback/foreign_transformer.dart
diff --git a/lib/src/barback/foreign_transformer.dart b/lib/src/barback/foreign_transformer.dart
index 5ec7734097863fd115e7f453dc73bc807bb7b2f1..147dbddbb3dab81ca35b1c111adfdb3315e5686a 100644
--- a/lib/src/barback/foreign_transformer.dart
+++ b/lib/src/barback/foreign_transformer.dart
@@ -123,9 +123,11 @@ class _ForeignGroup implements TransformerGroup {
   final String _toString;
 
   _ForeignGroup(TransformerConfig config, Map map)
-      : phases = map['phases'].map((phase) {
-          return phase.map((transformer) => deserializeTransformerLike(
-              transformer, config)).toList();
+      : phases = (map['phases'] as List).map((phase) {
+          return (phase as List)
+              .map((transformer) =>
+                  deserializeTransformerLike(transformer, config))
+              .toList();
         }).toList(),
         _toString = map['toString'];