Remove legacy restart code

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index e7a69b49df86409edae7ef45f84012b1dad0dda8..6f7b05971a7150a8e5564627ebb278423eac7da7 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -163,7 +163,6 @@ class Isolate : public BaseIsolate {
     // Internal message ids.
     kInterruptMsg = 10,     // Break in the debugger.
     kInternalKillMsg = 11,  // Like kill, but does not run exit listeners, etc.
-    kVMRestartMsg = 12,     // Sent to isolates when vm is restarting.
   };
   // The different Isolate API message priorities for ping and kill messages.
   enum LibMsgPriority {
@@ -530,7 +529,7 @@ class Isolate : public BaseIsolate {
     should_pause_post_service_request_ = should_pause_post_service_request;
   }
 
-  void PausePostRequest();
+  RawError* PausePostRequest();
 
   uword user_tag() const { return user_tag_; }
   static intptr_t user_tag_offset() { return OFFSET_OF(Isolate, user_tag_); }