Fix Isolate scopes in service isolate shutdown

runtime/vm/service_isolate.cc

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/service_isolate.h"
 
 #include "vm/compiler.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/isolate.h"
@@ skipping 341 matching lines @@
       Thread* T = Thread::Current();
       ASSERT(I == T->isolate());
       StackZone zone(T);
       HandleScope handle_scope(T);
       Error& error = Error::Handle(Z);
       error = I->object_store()->sticky_error();
       if (!error.IsNull() && !error.IsUnwindError()) {
         OS::PrintErr("vm-service: Error: %s\n", error.ToErrorCString());
       }
       Dart::RunShutdownCallback();
-
+    }
+    {
       // Shut the isolate down.
+      SwitchIsolateScope switch_scope(I);
       Dart::ShutdownIsolate();
     }

[siva, 2015/10/30 15:22:47]

Can we switch this to:

{
   ASSERT(Isolate::Current() == NULL);
   StartIsolateScope start_scope(I);
   {
      Thread* T ...
      .....
      .....
   }
   Dart::ShutdownIsolate();
}

StartIsolateScope itself works fine if the isolate is shutdown under it as there
is a check in Thread::ExitIsolate.

I would like to minimize usage of SwitchIsolateScope in the VM code as far as
possible.

[zra, 2015/10/30 16:48:40]

Discussed offline. It is also not possible to do ShutdownIsolate inside of a
StartIsolateScope.

     if (FLAG_trace_service) {
       OS::Print("vm-service: Shutdown.\n");
     }
     ServiceIsolate::FinishedExiting();
   }
 
   void RunMain(Isolate* I) {
     Thread* T = Thread::Current();
     ASSERT(I == T->isolate());
     StackZone zone(T);
@@ skipping 109 matching lines @@
   ASSERT(!result.IsNull());
   Dart_Port port = ILLEGAL_PORT;
   if (result.IsReceivePort()) {
     port = ReceivePort::Cast(result).Id();
   }
   ASSERT(port != ILLEGAL_PORT);
   ServiceIsolate::SetServicePort(port);
 }
 
 }  // namespace dart