Allow transformers to exclude and include assets.

sdk/lib/_internal/pub/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index b44bb555b43702ba7efccfcba060803ca122a666..274806179f9f21c8da393c183b24d0324c0802e2 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -162,18 +162,9 @@ class Pubspec {
             _error('"$field.$library" field must be a map, but was '
                 '"$configuration".');
           }
-
-          var reservedKeys = configuration.keys
-              .where((key) => key is String && key.startsWith(r'$'))
-              .map((key) => '"$key"');
-          if (reservedKeys.isNotEmpty) {
-            _error('"$field.$library" field cannot contain reserved '
-                '${pluralize('field', reservedKeys.length)} '
-                '${toSentence(reservedKeys)}.');
-          }
         }
 
-        var id = _wrapFormatException("transformer identifier",
+        var id = _wrapFormatException("transformer configuration",
             "$field.$library",
             () => new TransformerId.parse(library, configuration));