Add flag --support_debugger, controls emitting single stepping checks. Disable in --noopt, 6% gain.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
===================================================================
--- runtime/vm/flow_graph_compiler.cc	(revision 45525)
+++ runtime/vm/flow_graph_compiler.cc	(working copy)
@@ -54,6 +54,7 @@
 DECLARE_FLAG(int, reoptimization_counter_threshold);
 DECLARE_FLAG(int, stacktrace_every);
 DECLARE_FLAG(charp, stacktrace_filter);
+DECLARE_FLAG(bool, support_debugger);
 DECLARE_FLAG(bool, use_cha);
 DECLARE_FLAG(bool, use_field_guards);
 DECLARE_FLAG(bool, use_osr);
@@ -66,6 +67,7 @@
     FLAG_use_field_guards = false;
     FLAG_use_osr = false;
     FLAG_emit_edge_counters = false;
+    FLAG_support_debugger = false;
   }
 }