Enqueue superclasses instead of supertypes.

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 65cf5969b9de21349f44b598076923f8cd8589e4..239b902b820e53065b5d51aadeedf52cacb09947 100644
--- a/pkg/compiler/lib/src/types/type_mask.dart
+++ b/pkg/compiler/lib/src/types/type_mask.dart
@@ -88,8 +88,8 @@ abstract class TypeMask implements ReceiverMask {
 
   factory TypeMask.exact(ClassElement base, ClassWorld classWorld) {
     assert(invariant(base, classWorld.isInstantiated(base),
-        message: "Cannot create exact type mask for uninstantiated class "
-          "${base.name}"));
+        message: () => "Cannot create exact type mask for uninstantiated "
+                       "class $base.\n${classWorld.dump()}"));
     return new FlatTypeMask.exact(base);
   }
 
@@ -121,8 +121,8 @@ abstract class TypeMask implements ReceiverMask {
 
   factory TypeMask.nonNullExact(ClassElement base, ClassWorld classWorld) {
     assert(invariant(base, classWorld.isInstantiated(base),
-        message: "Cannot create exact type mask for "
-                 "uninstantiated class $base."));
+        message: () => "Cannot create exact type mask for "
+                 "uninstantiated class $base.\n${classWorld.dump()}"));
     return new FlatTypeMask.nonNullExact(base);
   }