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 0721fe2f91615dbc0fc9701028de5d05aec93ef9..2e958128abf5923c44823815fb176587b492390f 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -234,8 +234,10 @@ class Scavenger {
   void AllocateExternal(intptr_t size);
   void FreeExternal(intptr_t size);
 
-  uword FlushTLS() const;
-  void UnflushTLS(uword value) const;
+  uword FindTopOfSpace(Isolate* isolate) const;
+  void MakeALLTLABsIterable(Isolate* isolate) const;
+  void FlushTLS() const;
+  void AbandonAllTLABs();
   uword FirstObjectStart() const { return to_->start() | object_alignment_; }
 
  private:
@@ -334,6 +336,7 @@ class Scavenger {
 
   bool failed_to_promote_;
 
+  Mutex* space_lock_;
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;