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

runtime/vm/simulator_mips.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Declares a Simulator for MIPS instructions if we are not generating a native
 // MIPS binary. This Simulator allows us to run and debug MIPS code generation
 // on regular desktop machines.
 // Dart calls into generated code by "calling" the InvokeDartCode stub,
 // which will start execution in the Simulator or forwards to the real entry
 // on a MIPS HW platform.
@@ skipping 78 matching lines @@
 
   // Illegal memory access support.
   static bool IsIllegalAddress(uword addr) {
     return addr < 64*1024;
   }
   void HandleIllegalAccess(uword addr, Instr* instr);
 
   // Read and write memory.
   void UnalignedAccess(const char* msg, uword addr, Instr* instr);
 
+  // Handles a legal instruction that the simulator does not implement.
+  void UnimplementedInstruction(Instr* instr);
+
   bool OverflowFrom(int32_t alu_out,
                     int32_t left,
                     int32_t right,
                     bool addition);
 
   void set_pc(uword value) { pc_ = value; }
 
   void Format(Instr* instr, const char* format);
 
   inline int8_t ReadB(uword addr);
@@ skipping 12 matching lines @@
 
   void Execute();
   void ExecuteDelaySlot();
 
   friend class SimulatorDebugger;
 };
 
 }  // namespace dart
 
 #endif  // VM_SIMULATOR_MIPS_H_