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

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 36530)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -20,6 +20,7 @@
 
 namespace dart {
 
+DECLARE_FLAG(bool, emit_edge_counters);
 DECLARE_FLAG(int, optimization_counter_threshold);
 DECLARE_FLAG(bool, propagate_ic_data);
 DECLARE_FLAG(bool, throw_on_javascript_int_overflow);
@@ -5563,7 +5564,9 @@
 void TargetEntryInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   __ Bind(compiler->GetJumpLabel(this));
   if (!compiler->is_optimizing()) {
-    compiler->EmitEdgeCounter();
+    if (FLAG_emit_edge_counters) {
+      compiler->EmitEdgeCounter();
+    }
     // The deoptimization descriptor points after the edge counter code for
     // uniformity with ARM and MIPS, where we can reuse pattern matching
     // code that matches backwards from the end of the pattern.
@@ -5584,7 +5587,9 @@
 
 void GotoInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   if (!compiler->is_optimizing()) {
-    compiler->EmitEdgeCounter();
+    if (FLAG_emit_edge_counters) {
+      compiler->EmitEdgeCounter();
+    }
     // Add a deoptimization descriptor for deoptimizing instructions that
     // may be inserted before this instruction.  This descriptor points
     // after the edge counter for uniformity with ARM and MIPS, where we can