Fix crash with cyclic exports

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 6701 matching lines @@
   if (loaded_scripts() != Array::null()) {
     StorePointer(&raw_ptr()->loaded_scripts_, Array::null());
   }
 }
 
 
 // Lookup a name in the library's export namespace.
 RawObject* Library::LookupExport(const String& name) const {
   if (HasExports()) {
     const Array& exports = Array::Handle(this->exports());
+    // Break potential export cycle while looking up name.
+    StorePointer(&raw_ptr()->exports_, Object::empty_array().raw());
     Namespace& ns = Namespace::Handle();
     Object& obj = Object::Handle();
     for (int i = 0; i < exports.Length(); i++) {
       ns ^= exports.At(i);
       obj = ns.Lookup(name);
       if (!obj.IsNull()) {
-        return obj.raw();
+        break;
       }
     }
+    StorePointer(&raw_ptr()->exports_, exports.raw());
+    return obj.raw();
   }
   return Object::null();
 }
 
 
 RawObject* Library::LookupEntry(const String& name, intptr_t *index) const {
   Isolate* isolate = Isolate::Current();
   ReusableHandleScope reused_handles(isolate);
   Array& dict = reused_handles.ArrayHandle();
   dict ^= dictionary();
@@ skipping 8155 matching lines @@
 }
 
 
 void MirrorReference::PrintToJSONStream(JSONStream* stream, bool ref) const {
   stream->OpenObject();
   stream->CloseObject();
 }
 
 
 }  // namespace dart