Enqueue superclasses instead of supertypes.

pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
index 4bb654a97f8dd5ad7de866c73d60c7f5a0b1528f..58c7c8df313e07f5844660c410c16d7a82a8ff82 100644
--- a/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
@@ -454,6 +454,8 @@ class ConcreteTypeSystem extends TypeSystem<ConcreteType> {
       } else if (element == compiler.backend.intImplementation) {
         return new TypeMask.nonNullSubclass(compiler.backend.intImplementation,
                                             compiler.world);
+      } else if (!compiler.world.isInstantiated(element.declaration)) {
+        return new TypeMask.nonNullSubtype(element.declaration, compiler.world);
       } else {
         return new TypeMask.nonNullExact(element.declaration, compiler.world);
       }