Scavenge watched objects by scavenging their watchers.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 9767e03c6760ecca0f233b057a1b4a3ccda67af5..b5696a1f1b6ad781a5dff96f57e36ed603c778e0 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -122,18 +122,26 @@ class ScavengerVisitor : public ObjectPointerVisitor {
     if (IsForwarding(header)) {
       // Get the new location of the object.
       new_addr = ForwardedAddr(header);
-    } else {
-      if (raw_obj->IsWatched()) {
-        std::pair<DelaySet::iterator, DelaySet::iterator> ret;
-        // Visit all elements with a key equal to raw_obj.
-        ret = delay_set_.equal_range(raw_obj);
-        for (DelaySet::iterator it = ret.first; it != ret.second; ++it) {
-          // Visit through the associated WeakProperty at this time.
-          it->second->VisitPointers(this);
-        }
-        delay_set_.erase(ret.first, ret.second);
-        raw_obj->ClearWatchedBit();
+    } else if (raw_obj->IsWatched()) {
+      // Forward the object by scavenging its watchers.
+      raw_obj->ClearWatchedBit();
+      std::pair<DelaySet::iterator, DelaySet::iterator> ret;
+      // Visit all elements with a key equal to raw_obj.
+      ret = delay_set_.equal_range(raw_obj);
+      for (DelaySet::iterator it = ret.first; it != ret.second; ++it) {
+        // Scavenge the delayed WeakProperty.  These objects have been
+        // forwarded but have not been scavenged because their key
+        // object was not known to be reachable.  Now that the key
+        // object is known to be reachable we can scavenge the key and
+        // value pointers.
+        it->second->VisitPointers(this);
       }
+      delay_set_.erase(ret.first, ret.second);
+      // Reread the header word to get the new location of the object.
+      header = *reinterpret_cast<uword*>(raw_addr);
+      ASSERT(IsForwarding(header));
+      new_addr = ForwardedAddr(header);
+    } else {
       intptr_t size = raw_obj->Size();
       // Check whether object should be promoted.
       if (scavenger_->survivor_end_ <= raw_addr) {