Lazy peer API.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index cf5cff157cec7bb66ab0a7d709bf83567f89f786..9f6bbce078405106b6cfcfba2e973b5835f5b50d 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -378,6 +378,22 @@ void GCMarker::ProcessWeakProperty(RawWeakProperty* raw_weak,
 }
 
 
+void GCMarker::ProcessPeerReferents() {
+  std::map<RawObject*, void*>::iterator it = peer_->begin();
+  while (it != peer_->end()) {
+    RawObject* raw_obj = it->first;
+    ASSERT(raw_obj->IsHeapObject());
+    if (raw_obj->IsMarked()) {
+      // The object has survived.  Do nothing.
+      ++it;
+    } else {
+      // The object has become garbage.  Remove its record.
+      peer_->erase(it++);
+    }
+  }
+}
+
+
 void GCMarker::MarkObjects(Isolate* isolate,
                            PageSpace* page_space,
                            bool invoke_api_callbacks) {
@@ -390,6 +406,7 @@ void GCMarker::MarkObjects(Isolate* isolate,
   MarkingWeakVisitor mark_weak;
   IterateWeakRoots(isolate, &mark_weak, invoke_api_callbacks);
   mark.Finalize();
+  ProcessPeerReferents();
   Epilogue(isolate, invoke_api_callbacks);
 }