Add missing ExitIsolate to benchmark test.

runtime/vm/benchmark_test.cc

 // Copyright (c) 2012, 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/benchmark_test.h"
 
 #include "bin/builtin.h"
 #include "bin/file.h"
 
 #include "platform/assert.h"
@@ skipping 42 matching lines @@
 }
 
 
 //
 // Measure creation of core isolate from a snapshot.
 //
 BENCHMARK(CorelibIsolateStartup) {
   const int kNumIterations = 1000;
   Timer timer(true, "CorelibIsolateStartup");
   Isolate* isolate = Isolate::Current();
+  Thread::ExitIsolate();
   for (int i = 0; i < kNumIterations; i++) {
     timer.Start();
     TestCase::CreateTestIsolate();
     timer.Stop();
     Dart_ShutdownIsolate();
   }
   benchmark->set_score(timer.TotalElapsedTime() / kNumIterations);
   Thread::EnterIsolate(isolate);
 }
 
@@ skipping 521 matching lines @@
                           zone.GetZone());
     reader.ReadObject();
     free(buffer);
   }
   timer.Stop();
   int64_t elapsed_time = timer.TotalElapsedTime();
   benchmark->set_score(elapsed_time);
 }
 
 }  // namespace dart