Speed up kernel sanity checks.

pkg/kernel/lib/transformations/flags.dart

 library kernel.transformations.flags;
 
 import '../ast.dart';
 
 /// Flags summarizing the kinds of AST nodes contained in a given member or
 /// class, for speeding up transformations that only affect certain types of
 /// nodes.
 ///
 /// These are set by the frontend and the deserializer.
 class TransformerFlag {
   /// The class or member contains 'super' calls, that is, one of the AST nodes
   /// [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.
 }