Refactor Thread lifecycle interface, add Thread::Enter/ExitIsolate.

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 593 matching lines @@
                                                    NULL,
                                                    NULL,
                                                    NULL,
                                                    &error));
     if (isolate == NULL) {
       OS::PrintErr("vm-service: Isolate creation error: %s\n", error);
       ServiceIsolate::FinishedInitializing();
       return;
     }
 
-    Isolate::SetCurrent(NULL);
+    Thread::ExitIsolate();
 
     ServiceIsolate::ConstructExitMessageAndCache(isolate);
 
     RunMain(isolate);
 
     ServiceIsolate::FinishedInitializing();
 
     isolate->message_handler()->Run(Dart::thread_pool(),
                                     NULL,
                                     ShutdownIsolate,
@@ skipping 148 matching lines @@
     return result;
   }
   Dart_Handle source = GetSource(url_string);
   if (Dart_IsError(source)) {
     return source;
   }
   return Dart_LoadSource(library, url, source, 0, 0);
 }
 
 }  // namespace dart