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

runtime/vm/simulator_arm.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 ARM instructions if we are not generating a native
 // ARM binary. This Simulator allows us to run and debug ARM 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 ARM HW platform.
@@ skipping 112 matching lines @@
   // Registered breakpoints.
   Instr* break_pc_;
   int32_t break_instr_;
 
   // Illegal memory access support.
   static bool IsIllegalAddress(uword addr) {
     return addr < 64*1024;
   }
   void HandleIllegalAccess(uword addr, Instr* instr);
 
+  // Handles a legal instruction that the simulator does not implement.
+  void UnimplementedInstruction(Instr* instr);
+
   // Unsupported instructions use Format to print an error and stop execution.
   void Format(Instr* instr, const char* format);
 
   // Checks if the current instruction should be executed based on its
   // condition bits.
   bool ConditionallyExecute(Instr* instr);
 
   // Helper functions to set the conditional flags in the architecture state.
   void SetNZFlags(int32_t val);
   void SetCFlag(bool val);
@@ skipping 82 matching lines @@
   }
 
   friend class SimulatorDebugger;
   friend class SimulatorSetjmpBuffer;
   DISALLOW_COPY_AND_ASSIGN(Simulator);
 };
 
 }  // namespace dart
 
 #endif  // VM_SIMULATOR_ARM_H_