Fix incorrect assertion failures in noopt regarding removing classes that still have constants.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index c5b593e66014874277077296e8d81e8cdba2ac7b..b34af5460018d38e35ee9d4169ff90816a319dd3 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -446,6 +446,19 @@ void Heap::CollectAllGarbage() {
 }
 
 
+#if defined(DEBUG)
+void Heap::WaitForSweeperTasks() {
+  Thread* thread = Thread::Current();
+  {
+    MonitorLocker ml(old_space_.tasks_lock());
+    while (old_space_.tasks() > 0) {
+      ml.WaitWithSafepointCheck(thread);
+    }
+  }
+}
+#endif
+
+
 bool Heap::ShouldPretenure(intptr_t class_id) const {
   if (class_id == kOneByteStringCid) {
     return pretenure_policy_ > 0;