Reland r24563 and r24564 with fixes cumbersome API leading to leaks.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
===================================================================
--- runtime/vm/scavenger.h	(revision 24817)
+++ runtime/vm/scavenger.h	(working copy)
@@ -5,8 +5,6 @@
 #ifndef VM_SCAVENGER_H_
 #define VM_SCAVENGER_H_
 
-#include <map>
-
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
@@ -89,12 +87,6 @@
 
   void WriteProtect(bool read_only);
 
-  void SetPeer(RawObject* raw_obj, void* peer);
-
-  void* GetPeer(RawObject* raw_obj);
-
-  int64_t PeerCount() const;
-
  private:
   // Ids for time and data records in Heap::GCStats.
   enum {
@@ -145,7 +137,7 @@
     return end_ < to_->end();
   }
 
-  void ProcessPeerReferents();
+  void ProcessWeakTables();
 
   VirtualMemory* space_;
   MemoryRegion* to_;
@@ -153,9 +145,6 @@
 
   Heap* heap_;
 
-  typedef std::map<RawObject*, void*> PeerTable;
-  PeerTable peer_table_;
-
   // Current allocation top and end. These values are being accessed directly
   // from generated code.
   uword top_;