Fix for issue 25236

runtime/vm/gc_marker.cc

 // Copyright (c) 2011, 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/gc_marker.h"
 
 #include <map>
 #include <utility>
 #include <vector>
 
@@ skipping 577 matching lines @@
         marking_stack_(marking_stack),
         delay_set_(delay_set),
         barrier_(barrier),
         collect_code_(collect_code),
         task_index_(task_index),
         num_tasks_(num_tasks),
         num_busy_(num_busy) {
   }
 
   virtual void Run() {
-    Thread::EnterIsolateAsHelper(isolate_, true);
+    bool result = Thread::EnterIsolateAsHelper(isolate_, true);
+    ASSERT(result);
     {
       StackZone stack_zone(Thread::Current());
       Zone* zone = stack_zone.GetZone();
       SkippedCodeFunctions* skipped_code_functions =
           collect_code_ ? new(zone) SkippedCodeFunctions() : NULL;
       SyncMarkingVisitor visitor(isolate_, heap_, page_space_, marking_stack_,
                                  delay_set_, skipped_code_functions);
       // Phase 1: Iterate over roots and drain marking stack in tasks.
       marker_->IterateRoots(isolate_, &visitor, task_index_, num_tasks_);
       do {
@@ skipping 131 matching lines @@
       barrier.Exit();
     }
     delay_set.ClearReferences();
     ProcessWeakTables(page_space);
     ProcessObjectIdTable(isolate);
   }
   Epilogue(isolate, invoke_api_callbacks);
 }
 
 }  // namespace dart