Cache isolate in the StackTrace object to that we don't call

runtime/vm/zone.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/zone.h"
 
 #include "vm/assert.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
 #include "vm/os.h"
@@ skipping 205 matching lines @@
 #endif
 
 
 Zone::Zone()
     : zone_(),
       handles_(),
       previous_(NULL) {
   // Assert that there is no current zone as we only want to scope
   // zones when transitioning from generated dart code to dart VM
   // runtime code.
-  Isolate* isolate = Isolate::Current();
-  previous_ = isolate->current_zone();
-  Isolate::Current()->set_current_zone(this);
+  previous_ = isolate()->current_zone();
+  isolate()->set_current_zone(this);
 }
 
 
 Zone::~Zone() {
-  Isolate* isolate = Isolate::Current();
-  ASSERT(isolate->current_zone() == this);
-  isolate->set_current_zone(previous_);
+  ASSERT(isolate()->current_zone() == this);
+  isolate()->set_current_zone(previous_);
 }
 
 
 void Zone::VisitObjectPointers(ObjectPointerVisitor* visitor) {
   Zone* zone = this;
   while (zone != NULL) {
     zone->handles()->VisitObjectPointers(visitor);
     zone = zone->previous_;
   }
 }
 
 }  // namespace dart