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

runtime/vm/stub_code_arm64.cc

Index: runtime/vm/stub_code_arm64.cc
===================================================================
--- runtime/vm/stub_code_arm64.cc	(revision 36530)
+++ runtime/vm/stub_code_arm64.cc	(working copy)
@@ -26,6 +26,8 @@
     "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.
 //   SP : address of last argument in argument array.
@@ -1310,19 +1312,21 @@
   }
 #endif  // DEBUG
 
-  // Check single stepping.
-  Label not_stepping;
-  __ LoadFieldFromOffset(R6, CTX, Context::isolate_offset(), kNoPP);
-  __ LoadFromOffset(
-      R6, R6, Isolate::single_step_offset(), kNoPP, kUnsignedByte);
-  __ CompareRegisters(R6, ZR);
-  __ 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;
+    __ LoadFieldFromOffset(R6, CTX, Context::isolate_offset(), kNoPP);
+    __ LoadFromOffset(
+        R6, R6, Isolate::single_step_offset(), kNoPP, kUnsignedByte);
+    __ CompareRegisters(R6, ZR);
+    __ b(&not_stepping, EQ);
+    __ EnterStubFrame();
+    __ Push(R5);  // Preserve IC data.
+    __ CallRuntime(kSingleStepHandlerRuntimeEntry, 0);
+    __ Pop(R5);
+    __ LeaveStubFrame();
+    __ Bind(&not_stepping);
+  }
 
   // Load arguments descriptor into R4.
   __ LoadFieldFromOffset(R4, R5, ICData::arguments_descriptor_offset(), kNoPP);