Improve debugger in MIPS simulator.

runtime/vm/simulator_mips.h

Index: runtime/vm/simulator_mips.h
===================================================================
--- runtime/vm/simulator_mips.h	(revision 42124)
+++ runtime/vm/simulator_mips.h	(working copy)
@@ -90,6 +90,9 @@
   // Accessor to the internal simulator stack top.
   uword StackTop() const;
 
+  // Accessor to the instruction counter.
+  intptr_t get_icount() const { return icount_; }
+
   // The isolate's top_exit_frame_info refers to a Dart frame in the simulator
   // stack. The simulator's top_exit_frame_info refers to a C++ frame in the
   // native stack.
@@ -177,11 +180,6 @@
   // 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);