Update assertion regarding supertypes.

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 3318 matching lines @@
           Elements.constructConstructorName(target.name, selector);
     }
     return new SynthesizedConstructorElementX.forwarding(constructorName,
                                                          constructor,
                                                          target);
   }
 
   void doApplyMixinTo(MixinApplicationElement mixinApplication,
                       DartType supertype,
                       DartType mixinType) {
-    assert(mixinApplication.supertype == null);
-    mixinApplication.supertype = supertype;
+    Node node = mixinApplication.parseNode(compiler);
 
-    Node node = mixinApplication.parseNode(compiler);
+    if (mixinApplication.supertype != null) {
+      // [supertype] is not null if there was a cycle.
+      assert(invariant(node, compiler.compilationFailed));
+      supertype = mixinApplication.supertype;
+      assert(invariant(node, supertype.element == compiler.objectClass));
+    } else {
+      mixinApplication.supertype = supertype;
+    }
+
     // Named mixin application may have an 'implements' clause.
     NamedMixinApplication namedMixinApplication =
         node.asNamedMixinApplication();
     Link<DartType> interfaces = (namedMixinApplication != null)
         ? resolveInterfaces(namedMixinApplication.interfaces,
                             namedMixinApplication.superclass)
         : const Link<DartType>();
 
     // The class that is the result of a mixin application implements
     // the interface of the class that was mixed in so always prepend
@@ skipping 670 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }