Disable an expensive GC assert unless heap verification also enabled.

runtime/vm/gc_marker.cc

 // Copyright (c) 2011, 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/gc_marker.h"
 
 #include <map>
 #include <utility>
 
 #include "vm/allocation.h"
@@ skipping 146 matching lines @@
     for (; it != delay_set_.end(); ++it) {
       WeakProperty::Clear(it->second);
     }
   }
 
   void set_update_store_buffers(bool val) { update_store_buffers_ = val; }
 
  private:
   void MarkAndPush(RawObject* raw_obj) {
     ASSERT(raw_obj->IsHeapObject());
-    ASSERT(page_space_->Contains(RawObject::ToAddr(raw_obj)));
+    ASSERT((FLAG_verify_before_gc || FLAG_verify_before_gc) ?
+           page_space_->Contains(RawObject::ToAddr(raw_obj)) :
+           true);
 
     // Mark the object and push it on the marking stack.
     ASSERT(!raw_obj->IsMarked());
     RawClass* raw_class = isolate()->class_table()->At(raw_obj->GetClassId());
     raw_obj->SetMarkBit();
     if (raw_obj->IsWatched()) {
       std::pair<DelaySet::iterator, DelaySet::iterator> ret;
       // Visit all elements with a key equal to raw_obj.
       ret = delay_set_.equal_range(raw_obj);
       for (DelaySet::iterator it = ret.first; it != ret.second; ++it) {
@@ skipping 208 matching lines @@
   IterateRoots(isolate, &mark, !invoke_api_callbacks);
   DrainMarkingStack(isolate, &mark);
   IterateWeakReferences(isolate, &mark);
   MarkingWeakVisitor mark_weak;
   IterateWeakRoots(isolate, &mark_weak, invoke_api_callbacks);
   mark.Finalize();
   Epilogue(isolate, invoke_api_callbacks);
 }
 
 }  // namespace dart