Avoid eager enqueueing from resolution

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library elements.modelx;
 
 import '../common/resolution.dart' show
     Resolution,
     Parsing;
 import '../compiler.dart' show
@@ skipping 2575 matching lines @@
     }
     return null;
   }
 
   /**
    * Returns the super class, if any.
    *
    * 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;
   }
 
   void forEachBackendMember(void f(Element member)) {
     backendMembers.forEach(f);
   }
 
   bool implementsFunction(Compiler compiler) {
     return asInstanceOf(compiler.functionClass) != null || callType != null;
   }
@@ skipping 496 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }