Remove default isolate usage from almost all tests

src/v8threads.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 463 matching lines @@
 void ContextSwitcher::Run() {
   while (keep_going_) {
     OS::Sleep(sleep_ms_);
     isolate()->stack_guard()->Preempt();
   }
 }
 
 
 // Acknowledge the preemption by the receiving thread.
 void ContextSwitcher::PreemptionReceived() {
-  ASSERT(Locker::IsLocked(i::Isolate::GetDefaultIsolateForLocking()));
   // There is currently no accounting being done for this. But could be in the
   // future, which is why we leave this in.
 }
 
 
 }  // namespace internal
 }  // namespace v8