Add missing import: fix warning introduced in last CL

pkg/kernel/test/closures/suite.dart

Index: pkg/kernel/test/closures/suite.dart
diff --git a/pkg/kernel/test/closures/suite.dart b/pkg/kernel/test/closures/suite.dart
index 60bbc49e293b31aeabe8876f107099e2bfecbbc6..fdb8c5cb03169d7ab39187b4988948a2b9ebfea0 100644
--- a/pkg/kernel/test/closures/suite.dart
+++ b/pkg/kernel/test/closures/suite.dart
@@ -13,6 +13,8 @@ import 'package:testing/testing.dart'
 
 import 'package:kernel/ast.dart' show Program, Library;
 
+import 'package:kernel/target/targets.dart' show Target;
+
 import 'package:kernel/transformations/closure_conversion.dart'
     as closure_conversion;