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

runtime/vm/allocation.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/allocation.h"
 
 #include "vm/assert.h"
 #include "vm/isolate.h"
 #include "vm/zone.h"
 
 namespace dart {
 
 StackResource::StackResource() {
-  Isolate* isolate = Isolate::Current();
-  previous_ = isolate->top_resource();
-  isolate->set_top_resource(this);
+  isolate_ = Isolate::Current();
+  previous_ = isolate_->top_resource();
+  isolate_->set_top_resource(this);
 }
 
 
 StackResource::~StackResource() {
-  Isolate* isolate = Isolate::Current();
-  StackResource* top = isolate->top_resource();
+  StackResource* top = isolate_->top_resource();
   ASSERT(top == this);
-  isolate->set_top_resource(previous_);
+  isolate_->set_top_resource(previous_);
 }
 
 ZoneAllocated::~ZoneAllocated() {
   UNREACHABLE();
 }
 
 void* ZoneAllocated::operator new(uword size) {
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate != NULL);
   ASSERT(isolate->current_zone() != NULL);
   return reinterpret_cast<void*>(isolate->current_zone()->Allocate(size));
 }
 
 }  // namespace dart