Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index a13bcce75a27f8ceee815933cd333ac69f8ebe6e..34810a109fe21f4382c01d7ad88892b38b329592 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -157,6 +157,7 @@ class ScavengerVisitor : public ObjectPointerVisitor {
           NOT_IN_PRODUCT(class_table->UpdateAllocatedOld(cid, size));
         } else {
           // Promotion did not succeed. Copy into the to space instead.
+          scavenger_->failed_to_promote_ = true;
           new_addr = scavenger_->TryAllocate(size);
           NOT_IN_PRODUCT(class_table->UpdateLiveNew(cid, size));
         }
@@ -338,7 +339,8 @@ Scavenger::Scavenger(Heap* heap,
       delayed_weak_properties_(NULL),
       gc_time_micros_(0),
       collections_(0),
-      external_size_(0) {
+      external_size_(0),
+      failed_to_promote_(false) {
   // Verify assumptions about the first word in objects which the scavenger is
   // going to use for forwarding pointers.
   ASSERT(Object::tags_offset() == 0);
@@ -796,6 +798,8 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
   ASSERT(!scavenging_);
   scavenging_ = true;
 
+  failed_to_promote_ = false;
+
   PageSpace* page_space = heap_->old_space();
   NoSafepointScope no_safepoints;
 
@@ -905,4 +909,23 @@ void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(external_size_ >= 0);
 }
 
+
+void Scavenger::Evacuate() {
+  // We need a safepoint here to prevent allocation right before or right after
+  // the scavenge.
+  // The former can introduce an object that we might fail to collect.
+  // The latter means even if the scavenge promotes every object in the new
+  // space, the new allocation means the space is not empty,
+  // causing the assertion below to fail.
+  SafepointOperationScope scope(Thread::Current());
+
+  // Forces the next scavenge to promote all the objects in the new space.
+  survivor_end_ = top_;
+  Scavenge();
+
+  // It is possible for objects to stay in the new space
+  // if the VM cannot create more pages for these objects.
+  ASSERT((UsedInWords() == 0) || failed_to_promote_);
+}
+
 }  // namespace dart