Fix --gc_at_alloc

runtime/vm/scavenger.cc

 // Copyright (c) 2011, 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/scavenger.h"
 
 #include <algorithm>
 #include <map>
 #include <utility>
 
@@ skipping 390 matching lines @@
   }
   heap_->RecordData(kStoreBufferEntries, entries);
   // Done iterating through old objects remembered in the store buffers.
   visitor->VisitingOldObject(NULL);
 }
 
 
 void Scavenger::IterateObjectIdTable(Isolate* isolate,
                                      ScavengerVisitor* visitor) {
   ObjectIdRing* ring = isolate->object_id_ring();
-  ASSERT(ring != NULL);
+  if (ring == NULL) {
+    // --gc_at_alloc can get us here before the ring has been initialized.
+    ASSERT(FLAG_gc_at_alloc);
+    return;
+  }
   ring->VisitPointers(visitor);
 }
 
 
 void Scavenger::IterateRoots(Isolate* isolate,
                              ScavengerVisitor* visitor,
                              bool visit_prologue_weak_persistent_handles) {
   int64_t start = OS::GetCurrentTimeMicros();
   isolate->VisitObjectPointers(visitor,
                                visit_prologue_weak_persistent_handles,
@@ skipping 247 matching lines @@
 }
 
 
 void Scavenger::WriteProtect(bool read_only) {
   space_->Protect(
       read_only ? VirtualMemory::kReadOnly : VirtualMemory::kReadWrite);
 }
 
 
 }  // namespace dart