Partially implement DBC profiler.

runtime/vm/simulator_mips.h

Index: runtime/vm/simulator_mips.h
diff --git a/runtime/vm/simulator_mips.h b/runtime/vm/simulator_mips.h
index 700b1f5e2ba25da6ba82aa8edb2e6b02de1d4c94..cbda735d9472c0cd96c6b3219388e3ae640d44b0 100644
--- a/runtime/vm/simulator_mips.h
+++ b/runtime/vm/simulator_mips.h
@@ -99,7 +99,7 @@ class Simulator {
   // Accessor to the instruction counter.
   uint64_t get_icount() const { return icount_; }
 
-  // The isolate's top_exit_frame_info refers to a Dart frame in the simulator
+  // The thread'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.
   uword top_exit_frame_info() const { return top_exit_frame_info_; }