Remove some dead fields from Isolate.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index f29f6666330b84b27306ff2ec38695791592b12a..e45cdb7898afc3845e3984ba5afb0d796190abe7 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -735,7 +735,6 @@ class Isolate : public BaseIsolate {
   RawCode* ic_miss_code_;
   ClassTable class_table_;
   bool single_step_;
-  bool skip_step_;  // skip the next single step.
 
   ThreadRegistry* thread_registry_;
   SafepointHandler* safepoint_handler_;
@@ -749,7 +748,6 @@ class Isolate : public BaseIsolate {
   uint64_t terminate_capability_;
   bool errors_fatal_;
   ObjectStore* object_store_;
-  uword top_exit_frame_info_;
   void* init_callback_data_;
   Dart_EnvironmentCallback environment_callback_;
   Dart_LibraryTagHandler library_tag_handler_;
@@ -784,10 +782,6 @@ class Isolate : public BaseIsolate {
     bool error_on_bad_override_;
   )
 
-  // Status support.
-  char* stacktrace_;
-  intptr_t stack_frame_index_;
-
   // Timestamps of last operation via service.
   int64_t last_allocationprofile_accumulator_reset_timestamp_;
   int64_t last_allocationprofile_gc_timestamp_;