Implement multitest expectations.

pkg/testing/lib/src/test_description.dart

Index: pkg/testing/lib/src/test_description.dart
diff --git a/pkg/testing/lib/src/test_description.dart b/pkg/testing/lib/src/test_description.dart
index 9e2495b5aeafa2826ae2636f1c1f580f8f03b668..9b1c5b3169b4069342326d761f2b947951e2567b 100644
--- a/pkg/testing/lib/src/test_description.dart
+++ b/pkg/testing/lib/src/test_description.dart
@@ -13,6 +13,10 @@ class TestDescription implements Comparable<TestDescription> {
   final File file;
   final Uri output;
 
+  /// If non-null, this is a generated multitest, and the set contains the
+  /// expected outcomes.
+  Set<String> multitestExpectations;
+
   TestDescription(this.root, this.file, {this.output});
 
   Uri get uri => file.uri;