Speed up kernel sanity checks.

pkg/kernel/lib/transformations/flags.dart

Index: pkg/kernel/lib/transformations/flags.dart
diff --git a/pkg/kernel/lib/transformations/flags.dart b/pkg/kernel/lib/transformations/flags.dart
index 09d55d914b6028a86fe7608de01133ef5ece1b0b..093ff23e0cce16b818cdb026f81cc17b75bf719e 100644
--- a/pkg/kernel/lib/transformations/flags.dart
+++ b/pkg/kernel/lib/transformations/flags.dart
@@ -12,6 +12,10 @@ class TransformerFlag {
   /// [SuperPropertyGet], [SuperPropertySet], [SuperMethodInvocation].
   static const int superCalls = 1 << 0;
 
+  /// Temporary flag used by sanity checks to indicate that the given member
+  /// has been seen.
+  static const int seenBySanityCheck = 1 << 1;
+
   // TODO(asgerf):  We could also add a flag for 'async' and will probably have
   // one for closures as well.
 }