Make --trace-loading great again.

runtime/bin/vmservice_impl.cc

Index: runtime/bin/vmservice_impl.cc
diff --git a/runtime/bin/vmservice_impl.cc b/runtime/bin/vmservice_impl.cc
index 7ff23d06d4977850eb0283f451f5ea510c991679..c41fe1b8185394327864235ecb5b5d2a748f8210 100644
--- a/runtime/bin/vmservice_impl.cc
+++ b/runtime/bin/vmservice_impl.cc
@@ -170,7 +170,8 @@ bool VmService::LoadForGenPrecompiled() {
 bool VmService::Setup(const char* server_ip,
                       intptr_t server_port,
                       bool running_precompiled,
-                      bool dev_mode_server) {
+                      bool dev_mode_server,
+                      bool trace_loading) {
   Dart_Isolate isolate = Dart_CurrentIsolate();
   ASSERT(isolate != NULL);
   SetServerAddress("");
@@ -257,6 +258,12 @@ bool VmService::Setup(const char* server_ip,
       Dart_SetField(library, DartUtils::NewString("_isFuchsia"), is_fuchsia);
   SHUTDOWN_ON_ERROR(result);
 
+  if (trace_loading) {
+    result = Dart_SetField(library, DartUtils::NewString("_traceLoading"),
+                           Dart_True());
+    SHUTDOWN_ON_ERROR(result);
+  }
+
   // Get _getWatchSignalInternal from dart:io.
   Dart_Handle dart_io_str = Dart_NewStringFromCString(DartUtils::kIOLibURL);
   SHUTDOWN_ON_ERROR(dart_io_str);