Treat background finalization as another GC task so it won't run in parallel with the marker.

runtime/vm/dart_api_impl.cc

 // Copyright (c) 2013, 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 "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "lib/stacktrace.h"
@@ skipping 712 matching lines @@
   ApiState* state = Isolate::Current()->api_state();
   ASSERT(state->IsValidWeakPersistentHandle(handle));
 #endif
   return reinterpret_cast<FinalizablePersistentHandle*>(handle);
 }
 
 
 void FinalizablePersistentHandle::Finalize(
     Isolate* isolate, FinalizablePersistentHandle* handle) {
   if (!handle->raw()->IsHeapObject()) {
-    return;
+    return;  // Free handle.
   }
   Dart_WeakPersistentHandleFinalizer callback = handle->callback();
   ASSERT(callback != NULL);
   void* peer = handle->peer();
   Dart_WeakPersistentHandle object = handle->apiHandle();
   (*callback)(isolate->init_callback_data(), object, peer);
   ApiState* state = isolate->api_state();
   ASSERT(state != NULL);
   state->weak_persistent_handles().FreeHandle(handle);
 }
@@ skipping 283 matching lines @@
 
 static Dart_WeakPersistentHandle AllocateFinalizableHandle(
     Thread* thread,
     Dart_Handle object,
     void* peer,
     intptr_t external_allocation_size,
     Dart_WeakPersistentHandleFinalizer callback) {
   REUSABLE_OBJECT_HANDLESCOPE(thread);
   Object& ref = thread->ObjectHandle();
   ref = Api::UnwrapHandle(object);
+  if (!ref.raw()->IsHeapObject()) {
+    return NULL;
+  }
   FinalizablePersistentHandle* finalizable_ref =
       FinalizablePersistentHandle::New(thread->isolate(),
                                        ref,
                                        peer,
                                        callback,
                                        external_allocation_size);
   return finalizable_ref->apiHandle();
 }
 
 
@@ skipping 5533 matching lines @@
 
 DART_EXPORT bool Dart_IsPrecompiledRuntime() {
 #if defined(DART_PRECOMPILED_RUNTIME)
   return true;
 #else
   return false;
 #endif
 }
 
 }  // namespace dart