Add extra tracing to Debug_InterruptIsolate test.

runtime/vm/dart.h

Index: runtime/vm/dart.h
diff --git a/runtime/vm/dart.h b/runtime/vm/dart.h
index 412eef2c8625d653c429efc589d7b476b115bf08..45922965be6479ce947c6cb9b0246b3089ce100c 100644
--- a/runtime/vm/dart.h
+++ b/runtime/vm/dart.h
@@ -43,6 +43,10 @@ class Dart : public AllStatic {
   static Isolate* vm_isolate() { return vm_isolate_; }
   static ThreadPool* thread_pool() { return thread_pool_; }
 
+  // Returns a timestamp for use in debugging output in milliseconds
+  // since start time.
+  static int64_t timestamp();
+
   static void set_pprof_symbol_generator(DebugInfo* value) {
     pprof_symbol_generator_ = value;
   }
@@ -68,6 +72,7 @@ class Dart : public AllStatic {
   static void WaitForIsolateShutdown();
 
   static Isolate* vm_isolate_;
+  static int64_t start_time_;
   static ThreadPool* thread_pool_;
   static DebugInfo* pprof_symbol_generator_;
   static ReadOnlyHandles* predefined_handles_;