Ignore deleted handles when processing weak roots.

runtime/vm/gc_marker.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/gc_marker.h"
 
 #include "vm/allocation.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
 #include "vm/pages.h"
@@ skipping 179 matching lines @@
 
 class MarkingWeakVisitor : public HandleVisitor {
  public:
   MarkingWeakVisitor() {
   }
 
   void VisitHandle(uword addr) {
     WeakPersistentHandle* handle =
         reinterpret_cast<WeakPersistentHandle*>(addr);
     RawObject* raw_obj = handle->raw();
-    ASSERT(raw_obj->IsHeapObject());
+    if (!raw_obj->IsHeapObject()) return;
     if (!raw_obj->IsMarked() && raw_obj->IsOldObject()) {
       WeakPersistentHandle::Finalize(handle);
     }
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MarkingWeakVisitor);
 };
 
 
@@ skipping 28 matching lines @@
   MarkingStack marking_stack;
   Prologue(isolate);
   MarkingVisitor mark(heap_, page_space, &marking_stack);
   IterateRoots(isolate, &mark);
   DrainMarkingStack(isolate, &mark);
   MarkingWeakVisitor mark_weak;
   IterateWeakRoots(isolate, &mark_weak);
 }
 
 }  // namespace dart