Add flag —enable-debugger (default true) in order to disable debugger single stepping code.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
===================================================================
--- runtime/vm/debugger.cc	(revision 36530)
+++ runtime/vm/debugger.cc	(working copy)
@@ -27,11 +27,12 @@
 
 namespace dart {
 
-DEFINE_FLAG(bool, verbose_debug, false, "Verbose debugger messages");
+DEFINE_FLAG(bool, enable_debugger, true, "Enables debugger step checks");
+DEFINE_FLAG(bool, show_invisible_frames, false,
+            "Show invisible frames in debugger stack traces");
 DEFINE_FLAG(bool, trace_debugger_stacktrace, false,
             "Trace debugger stacktrace collection");
-DEFINE_FLAG(bool, show_invisible_frames, false,
-            "Show invisible frames in debugger stack traces");
+DEFINE_FLAG(bool, verbose_debug, false, "Verbose debugger messages");
 
 
 Debugger::EventHandler* Debugger::event_handler_ = NULL;