Partially implement DBC profiler.

runtime/vm/simulator_arm64.h

Index: runtime/vm/simulator_arm64.h
diff --git a/runtime/vm/simulator_arm64.h b/runtime/vm/simulator_arm64.h
index 78c1812ac7217137a67db87371cb4e8374754f4e..de49dd90bd21214f5865d129ae9f330731668e6c 100644
--- a/runtime/vm/simulator_arm64.h
+++ b/runtime/vm/simulator_arm64.h
@@ -80,7 +80,7 @@ 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_; }