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

runtime/vm/stub_code_arm.cc

Index: runtime/vm/stub_code_arm.cc
===================================================================
--- runtime/vm/stub_code_arm.cc	(revision 36530)
+++ runtime/vm/stub_code_arm.cc	(working copy)
@@ -27,6 +27,7 @@
     "Set to true for debugging & verifying the slow paths.");
 DECLARE_FLAG(bool, trace_optimized_ic_calls);
 
+DECLARE_FLAG(bool, enable_debugger);
 
 // Input parameters:
 //   LR : return address.
@@ -1268,18 +1269,20 @@
   }
 #endif  // DEBUG
 
-  // Check single stepping.
-  Label not_stepping;
-  __ ldr(R6, FieldAddress(CTX, Context::isolate_offset()));
-  __ ldrb(R6, Address(R6, Isolate::single_step_offset()));
-  __ CompareImmediate(R6, 0);
-  __ b(&not_stepping, EQ);
-  __ EnterStubFrame();
-  __ Push(R5);  // Preserve IC data.
-  __ CallRuntime(kSingleStepHandlerRuntimeEntry, 0);
-  __ Pop(R5);
-  __ LeaveStubFrame();
-  __ Bind(&not_stepping);
+  if (FLAG_enable_debugger) {
+    // Check single stepping.
+    Label not_stepping;
+    __ ldr(R6, FieldAddress(CTX, Context::isolate_offset()));
+    __ ldrb(R6, Address(R6, Isolate::single_step_offset()));
+    __ CompareImmediate(R6, 0);
+    __ b(&not_stepping, EQ);
+    __ EnterStubFrame();
+    __ Push(R5);  // Preserve IC data.
+    __ CallRuntime(kSingleStepHandlerRuntimeEntry, 0);
+    __ Pop(R5);
+    __ LeaveStubFrame();
+    __ Bind(&not_stepping);
+  }
 
   // Load arguments descriptor into R4.
   __ ldr(R4, FieldAddress(R5, ICData::arguments_descriptor_offset()));