Prepares allocation for proper sync with mutator and bg threads.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 819652674f2e0052d4de6b744dc69cce1300c9b7..b9cccdd1804f2a817690c5270c7ed563697cb887 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -236,8 +236,10 @@ class Scavenger {
   void AllocateExternal(intptr_t size);
   void FreeExternal(intptr_t size);
 
+  uword FindTopOfSpace(Isolate* isolate) const;
   uword FlushTLS() const;
   void UnflushTLS(uword value) const;
+  void AbandonAllTLABs();
   uword FirstObjectStart() const { return to_->start() | object_alignment_; }
 
  private:
@@ -336,6 +338,7 @@ class Scavenger {
 
   bool failed_to_promote_;
 
+  Mutex* space_lock_;
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;