Add --trace_type_check_elimination flag for debugging purposes.

runtime/vm/flow_graph_builder.h

Index: runtime/vm/flow_graph_builder.h
===================================================================
--- runtime/vm/flow_graph_builder.h	(revision 9382)
+++ runtime/vm/flow_graph_builder.h	(working copy)
@@ -215,6 +215,12 @@
     AddInstruction(new DoInstr(computation));
   }
 
+  // Returns true if the run-time type check can be eliminated.
+  bool CanSkipTypeCheck(intptr_t token_pos,
+                        Value* value,
+                        const AbstractType& dst_type,
+                        const String& dst_name);
+
   // Shared global state.
   FlowGraphBuilder* owner_;