Partially implement DBC profiler.

runtime/vm/simulator_arm.h

Index: runtime/vm/simulator_arm.h
diff --git a/runtime/vm/simulator_arm.h b/runtime/vm/simulator_arm.h
index ce18ac00fe4ee42b297b9b9ba30a02cafd13115d..c163a623c392f1c1eb8f8c2a9674aec36b5bde54 100644
--- a/runtime/vm/simulator_arm.h
+++ b/runtime/vm/simulator_arm.h
@@ -83,7 +83,7 @@ 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_; }