Avoid creating invalid TypeMask.nonNullExact

pkg/compiler/lib/src/types/type_mask.dart

Index: pkg/compiler/lib/src/types/type_mask.dart
diff --git a/pkg/compiler/lib/src/types/type_mask.dart b/pkg/compiler/lib/src/types/type_mask.dart
index a290aa2fa448c523d5f24137c69295ea8553f486..2f5754fd880ea1d11a3078ad17afdc385ac7a772 100644
--- a/pkg/compiler/lib/src/types/type_mask.dart
+++ b/pkg/compiler/lib/src/types/type_mask.dart
@@ -122,7 +122,7 @@ abstract class TypeMask implements ReceiverConstraint {
   factory TypeMask.nonNullExact(ClassElement base, ClassWorld classWorld) {
     assert(invariant(base, classWorld.isInstantiated(base),
         message: () => "Cannot create exact type mask for "
-                 "uninstantiated class $base.\n${classWorld.dump()}"));
+                 "uninstantiated class $base.\n${classWorld.dump(base)}"));
     return new FlatTypeMask.nonNullExact(base);
   }