Minimize enter/exit isolate calls during startup and shutdown of service isolate.

runtime/vm/service_isolate.cc

Index: runtime/vm/service_isolate.cc
diff --git a/runtime/vm/service_isolate.cc b/runtime/vm/service_isolate.cc
index 98cbeb957f7b42abdc8278494681b91bb35100f9..6f16f4190f9bd32fb3e349ded3074841000873cc 100644
--- a/runtime/vm/service_isolate.cc
+++ b/runtime/vm/service_isolate.cc
@@ -255,7 +255,6 @@ void ServiceIsolate::MaybeMakeServiceIsolate(Isolate* I) {
 void ServiceIsolate::ConstructExitMessageAndCache(Isolate* I) {
   // Construct and cache exit message here so we can send it without needing an
   // isolate.
-  StartIsolateScope iso_scope(I);
   Thread* T = Thread::Current();
   ASSERT(I == T->isolate());
   ASSERT(I != NULL);
@@ -324,12 +323,12 @@ class RunServiceTask : public ThreadPool::Task {
       return;
     }
 
-
-    Thread::ExitIsolate();
-
-    ServiceIsolate::ConstructExitMessageAndCache(isolate);
-
-    RunMain(isolate);
+    {
+      ASSERT(Isolate::Current() == NULL);
+      StartIsolateScope start_scope(isolate);
+      ServiceIsolate::ConstructExitMessageAndCache(isolate);
+      RunMain(isolate);
+    }
 
     ServiceIsolate::FinishedInitializing();
 
@@ -348,6 +347,7 @@ class RunServiceTask : public ThreadPool::Task {
     {
       // Print the error if there is one.  This may execute dart code to
       // print the exception object, so we need to use a StartIsolateScope.
+      ASSERT(Isolate::Current() == NULL);
       StartIsolateScope start_scope(I);
       Thread* T = Thread::Current();
       ASSERT(I == T->isolate());
@@ -359,10 +359,8 @@ class RunServiceTask : public ThreadPool::Task {
         OS::PrintErr("vm-service: Error: %s\n", error.ToErrorCString());
       }
       Dart::RunShutdownCallback();
-    }
-    {
+
       // Shut the isolate down.
-      SwitchIsolateScope switch_scope(I);
       Dart::ShutdownIsolate();
     }
     if (FLAG_trace_service) {
@@ -376,8 +374,6 @@ class RunServiceTask : public ThreadPool::Task {
       // TODO(24651): Remove this.
       return;
     }
-
-    StartIsolateScope iso_scope(I);
     Thread* T = Thread::Current();
     ASSERT(I == T->isolate());
     StackZone zone(T);