Puts TLABs back into the build and fixes assert failure.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index d97a4cfa1548eb9d7058dd4b76b3983f69f2e772..c32e7e63451358def58842205b5803762b5a8548 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2563,10 +2563,6 @@ Thread* Isolate::ScheduleThread(bool is_mutator, bool bypass_safepoint) {
     os_thread->set_thread(thread);
     if (is_mutator) {
       scheduled_mutator_thread_ = thread;
-      if (this != Dart::vm_isolate()) {
-        scheduled_mutator_thread_->set_top(heap()->new_space()->top());
-        scheduled_mutator_thread_->set_end(heap()->new_space()->end());
-      }
     }
     Thread::SetCurrent(thread);
     os_thread->EnableThreadInterrupts();
@@ -2605,12 +2601,6 @@ void Isolate::UnscheduleThread(Thread* thread,
   os_thread->set_thread(NULL);
   OSThread::SetCurrent(os_thread);
   if (is_mutator) {
-    if (this != Dart::vm_isolate()) {
-      heap()->new_space()->set_top(scheduled_mutator_thread_->top_);
-      heap()->new_space()->set_end(scheduled_mutator_thread_->end_);
-    }
-    scheduled_mutator_thread_->top_ = 0;
-    scheduled_mutator_thread_->end_ = 0;
     scheduled_mutator_thread_ = NULL;
   }
   thread->isolate_ = NULL;