Fix flaky disassembler crashes: do not call FindObject while concurrently sweeping.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
===================================================================
--- runtime/vm/heap.cc	(revision 43676)
+++ runtime/vm/heap.cc	(working copy)
@@ -253,6 +253,11 @@
 
 
 RawObject* Heap::FindOldObject(FindObjectVisitor* visitor) const {
+  // Wait for any concurrent GC tasks to finish before walking.
+  MonitorLocker ml(old_space_->tasks_lock());
+  while (old_space_->tasks() > 0) {
+    ml.Wait();
+  }
   return old_space_->FindObject(visitor, HeapPage::kData);
 }