Moves the top_ and end_ words of the Scavenger into mutator thread.

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index 7a98f4542b6bc3e76a8d11c5543b6e5ebeb0c7cf..e3761cd753a50022c97d302acb52acf95dff8639 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -365,6 +365,21 @@ class Thread : public BaseThread {
   Heap* heap() const { return heap_; }
   static intptr_t heap_offset() { return OFFSET_OF(Thread, heap_); }
 
+  void set_top(uword value) {
+    ASSERT(heap_ != NULL);
+    top_ = value;
+  }
+  void set_end(uword value) {
+    ASSERT(heap_ != NULL);
+    end_ = value;
+  }
+
+  uword top() { return top_; }
+  uword end() { return end_; }
+
+  static intptr_t top_offset() { return OFFSET_OF(Thread, top_); }
+  static intptr_t end_offset() { return OFFSET_OF(Thread, end_); }
+
   int32_t no_handle_scope_depth() const {
 #if defined(DEBUG)
     return no_handle_scope_depth_;
@@ -695,6 +710,8 @@ class Thread : public BaseThread {
   uword stack_overflow_flags_;
   Isolate* isolate_;
   Heap* heap_;
+  uword top_;
+  uword end_;
   uword top_exit_frame_info_;
   StoreBufferBlock* store_buffer_block_;
   uword vm_tag_;