Cache snapshots of (mostly) immutable transformer phases.

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

Index: sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart b/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
index 5d111ab034f1cea0f6e8ceb030bc6efb6bd93a6b..faadc6b9b48ca8b9eb08c83684f4f6a539e96d65 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
@@ -72,7 +72,11 @@ class TransformerId {
 
   int get hashCode => package.hashCode ^ path.hashCode;
 
-  String toString() => path == null ? package : '$package/$path';
+  /// Returns a serialized form of [this] that can be passed to
+  /// [new TransformerId.parse].
+  String serialize() => path == null ? package : '$package/$path';
+
+  String toString() => serialize();
 
   /// Returns the asset id for the library identified by this transformer id.
   ///