Puts TLABs back into the build and fixes assert failure.

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index e3761cd753a50022c97d302acb52acf95dff8639..002b78a5410ad148ec0b6577f27a56d4b2950a64 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -366,17 +366,17 @@ class Thread : public BaseThread {
   static intptr_t heap_offset() { return OFFSET_OF(Thread, heap_); }
 
   void set_top(uword value) {
-    ASSERT(heap_ != NULL);
     top_ = value;
   }
   void set_end(uword value) {
-    ASSERT(heap_ != NULL);
     end_ = value;
   }
 
   uword top() { return top_; }
   uword end() { return end_; }
 
+  bool HasActiveTLAB() { return end_ > 0; }
+
   static intptr_t top_offset() { return OFFSET_OF(Thread, top_); }
   static intptr_t end_offset() { return OFFSET_OF(Thread, end_); }