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