Avoid eager enqueueing from resolution

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 18e76d1314e8ccda1d24427b9e8e7788bb808a31..7ef3d1e36d73166b88b0712e892943fcf592bdc5 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -2595,7 +2595,8 @@ abstract class BaseClassElementX extends ElementX
    * The returned element may not be resolved yet.
    */
   ClassElement get superclass {
-    assert(supertypeLoadState == STATE_DONE);
+    assert(invariant(this, supertypeLoadState == STATE_DONE,
+        message: "Superclass has not been computed for $this."));
     return supertype == null ? null : supertype.element;
   }