Set the supertype to avoid loop.

dart/sdk/lib/_internal/compiler/implementation/resolution/members.dart

 // Copyright (c) 2012, 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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element get currentElement;
   Set<Node> get superUses;
 
@@ skipping 467 matching lines @@
    */
   void loadSupertypes(ClassElement cls, Spannable from) {
     compiler.withCurrentElement(cls, () => measure(() {
       if (cls.supertypeLoadState == STATE_DONE) return;
       if (cls.supertypeLoadState == STATE_STARTED) {
         compiler.reportErrorCode(from, MessageKind.CYCLIC_CLASS_HIERARCHY,
                                  {'className': cls.name});
         cls.supertypeLoadState = STATE_DONE;
         cls.allSupertypes = const Link<DartType>().prepend(
             compiler.objectClass.computeType(compiler));
-        // TODO(ahe): We should also set cls.supertype here to avoid
-        // creating a malformed class hierarchy.
+        cls.supertype = cls.allSupertypes.head;
         return;
       }
       cls.supertypeLoadState = STATE_STARTED;
       compiler.withCurrentElement(cls, () {
         // TODO(ahe): Cache the node in cls.
         cls.parseNode(compiler).accept(
             new ClassSupertypeResolver(compiler, cls));
         if (cls.supertypeLoadState != STATE_DONE) {
           cls.supertypeLoadState = STATE_DONE;
         }
@@ skipping 2727 matching lines @@
     compiler.ensure(element != null);
     compiler.ensure(element.resolutionState == STATE_STARTED);
 
     InterfaceType type = element.computeType(compiler);
     scope = new TypeDeclarationScope(scope, element);
     // TODO(ahe): It is not safe to call resolveTypeVariableBounds yet.
     // As a side-effect, this may get us back here trying to
     // resolve this class again.
     resolveTypeVariableBounds(node.typeParameters);
 
-    // Setup the supertype for the element.
-    assert(element.supertype == null);
-    if (node.superclass != null) {
+    // Setup the supertype for the element (if there is a cycle in the
+    // class hierarchy, it has already been set to Object).
+    if (element.supertype == null && node.superclass != null) {
       MixinApplication superMixin = node.superclass.asMixinApplication();
       if (superMixin != null) {
         DartType supertype = resolveSupertype(element, superMixin.superclass);
         Link<Node> link = superMixin.mixins.nodes;
         while (!link.isEmpty) {
           supertype = applyMixin(supertype, resolveType(link.head));
           link = link.tail;
         }
         element.supertype = supertype;
       } else {
@@ skipping 774 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }