Fixes the regression caused by 7568e1f18e.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 55720c97d681dcebdbff57a779d9c3dc8158d180..498c254ad1ff182ee1e88a18f0146b05e1e808ac 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2575,7 +2575,8 @@ Thread* Isolate::ScheduleThread(bool is_mutator, bool bypass_safepoint) {
     if (is_mutator) {
       mutator_thread_ = thread;
       if ((Dart::vm_isolate() != NULL) &&

[rmacnak, 2017/08/07 21:29:35]

Isn't this always a no-op now? Either the Thread still has 0/0 from when it was
constructed, has 0/0 from losing its TLAB in a GC, or has a TLAB we're going to
keep.

[danunez, 2017/08/07 23:09:59]

This is a no-op. This will be deleted.

-          (heap() != Dart::vm_isolate()->heap())) {
+          (heap() != Dart::vm_isolate()->heap()) &&
+          (!mutator_thread_->HasActiveTLAB())) {
         mutator_thread_->set_top(0);
         mutator_thread_->set_end(0);
       }
@@ -2617,13 +2618,6 @@ void Isolate::UnscheduleThread(Thread* thread,
   os_thread->set_thread(NULL);
   OSThread::SetCurrent(os_thread);
   if (is_mutator) {
-    if ((Dart::vm_isolate() != NULL) &&
-        (heap() != Dart::vm_isolate()->heap())) {
-      if (mutator_thread_->HasActiveTLAB()) {
-        heap()->AbandonRemainingTLAB(mutator_thread_);
-      }
-    }
-    ASSERT(!mutator_thread_->HasActiveTLAB());
     mutator_thread_ = NULL;
   }
   thread->isolate_ = NULL;