Drops into Simulator Debugger only in a session that is already interactive.

runtime/vm/simulator_arm.h

Index: runtime/vm/simulator_arm.h
===================================================================
--- runtime/vm/simulator_arm.h	(revision 20300)
+++ runtime/vm/simulator_arm.h	(working copy)
@@ -130,6 +130,19 @@
   }
   void HandleIllegalAccess(uword addr, Instr* instr);
 
+  // Handles a legal instruction that the simulator does not implement.
+  void UnimplementedInstruction(Instr* instr);
+
+  // If something goes wrong (Unimplemented instruction, illegal access, etc.),
+  // this flag is used to decide whether to die or to drop into the simulator
+  // debugger. Its initial value is false. Then, when we enter the simulator
+  // debugger (for a user set breakpoint, etc.) it is set to true.
+  bool debugger_attached_;
+
+  // Accessors to set and check whether the simulator debugger is attached.
+  bool debugger_attached() const { return debugger_attached_; }
+  void set_debugger_attached(bool b) { debugger_attached_ = b; }
+
   // Unsupported instructions use Format to print an error and stop execution.
   void Format(Instr* instr, const char* format);