Set previous instruction pointers when building the graph.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index 1594c57c7985a5e952533c62b60c41493edbd9f3..70316163969f4ad92fb64acfcf5b6897c9e24352 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -3134,10 +3134,10 @@ void ConstantPropagator::Transform() {
         GotoInstr* jump = new GotoInstr(join);
         Instruction* previous = branch->previous();
         branch->set_previous(NULL);
-        previous->set_next(jump);
+        previous->LinkTo(jump);
         // Replace the false target entry with the new join entry. We will
         // recompute the dominators after this pass.
-        join->set_next(next);
+        join->LinkTo(next);
       }
     }
   }