Makes new space iterable by filling up the mutator's TLAB

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index e3761cd753a50022c97d302acb52acf95dff8639..6f3100279f926700f702e08d294bfaed0e5e1376 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -377,6 +377,8 @@ class Thread : public BaseThread {
   uword top() { return top_; }
   uword end() { return end_; }
 
+  bool HasActiveTLAB() { return end_ > 0; }
+
   static intptr_t top_offset() { return OFFSET_OF(Thread, top_); }
   static intptr_t end_offset() { return OFFSET_OF(Thread, end_); }