Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 4998b881cfdc1c4876e1cbf79b991a4853aaa9c1..39cc7ba39c2467c90d301cf560b31b7985e17ae1 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -38,7 +38,8 @@ Heap::Heap(Isolate* isolate,
       barrier_done_(new Monitor()),
       read_only_(false),
       gc_new_space_in_progress_(false),
-      gc_old_space_in_progress_(false) {
+      gc_old_space_in_progress_(false),
+      debug_gc_in_progress_(false) {
   UpdateGlobalMaxUsed();
   for (int sel = 0; sel < kNumWeakSelectors; sel++) {
     new_weak_tables_[sel] = new WeakTable();
@@ -375,7 +376,11 @@ void Heap::CollectNewSpaceGarbage(Thread* thread,
     VMTagScope tagScope(thread, VMTag::kGCNewSpaceTagId);
     TIMELINE_FUNCTION_GC_DURATION(thread, "CollectNewGeneration");
     NOT_IN_PRODUCT(UpdateClassHeapStatsBeforeGC(kNew));
-    new_space_.Scavenge(invoke_api_callbacks);
+    if (debug_gc_in_progress_) {
+      new_space_.Evacuate();
+    } else {
+      new_space_.Scavenge(invoke_api_callbacks);
+    }
     NOT_IN_PRODUCT(isolate()->class_table()->UpdatePromoted());
     RecordAfterGC(kNew);
     PrintStats();
@@ -444,8 +449,10 @@ void Heap::CollectGarbage(Space space) {
 
 void Heap::CollectAllGarbage() {
   Thread* thread = Thread::Current();
+  debug_gc_in_progress_ = true;

[rmacnak, 2017/06/19 19:29:00]

Can this be

new_space_.Evacuate();
CollectOldSpaceGarbage(thread, kInvokeApiCallbacks, kFull);

? Without introducing a new state on the heap?

   CollectNewSpaceGarbage(thread, kInvokeApiCallbacks, kFull);
   CollectOldSpaceGarbage(thread, kInvokeApiCallbacks, kFull);
+  debug_gc_in_progress_ = false;
 }