Fix bugs in load elimination and type propagation.

runtime/vm/flow_graph_type_propagator.h

Index: runtime/vm/flow_graph_type_propagator.h
===================================================================
--- runtime/vm/flow_graph_type_propagator.h	(revision 28924)
+++ runtime/vm/flow_graph_type_propagator.h	(working copy)
@@ -12,11 +12,13 @@
 
 class FlowGraphTypePropagator : public FlowGraphVisitor {
  public:
+  static void Propagate(FlowGraph* flow_graph);
+
+ private:
   explicit FlowGraphTypePropagator(FlowGraph* flow_graph);
 
   void Propagate();
 
- private:
   void PropagateRecursive(BlockEntryInstr* block);
   void HandleBranchOnNull(BlockEntryInstr* block);