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

runtime/vm/heap.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 235 matching lines @@
 RawInstructions* Heap::FindObjectInCodeSpace(FindObjectVisitor* visitor) const {
   // Only executable pages can have RawInstructions objects.
   RawObject* raw_obj = old_space_->FindObject(visitor, HeapPage::kExecutable);
   ASSERT((raw_obj == Object::null()) ||
          (raw_obj->GetClassId() == kInstructionsCid));
   return reinterpret_cast<RawInstructions*>(raw_obj);
 }
 
 
 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);
 }
 
 
 RawObject* Heap::FindNewObject(FindObjectVisitor* visitor) const {
   return new_space_->FindObject(visitor);
 }
 
 
 RawObject* Heap::FindObject(FindObjectVisitor* visitor) const {
@@ skipping 463 matching lines @@
     heap->DisableGrowthControl();
 }
 
 
 NoHeapGrowthControlScope::~NoHeapGrowthControlScope() {
     Heap* heap = reinterpret_cast<Isolate*>(isolate())->heap();
     heap->SetGrowthControlState(current_growth_controller_state_);
 }
 
 }  // namespace dart