Provide the isolate with a trace buffer, but do not allocate one by default.

runtime/vm/isolate.cc

 // Copyright (c) 2013, 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/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 335 matching lines @@
       spawn_state_(NULL),
       is_runnable_(false),
       gc_prologue_callback_(NULL),
       gc_epilogue_callback_(NULL),
       defer_finalization_count_(0),
       deopt_context_(NULL),
       stacktrace_(NULL),
       stack_frame_index_(-1),
       cha_used_(false),
       object_id_ring_(NULL),
+      trace_buffer_(NULL),
       profiler_data_(NULL),
       thread_state_(NULL),
       tag_table_(GrowableObjectArray::null()),
       current_tag_(UserTag::null()),
       next_(NULL),
       REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_INITIALIZERS)
       REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_SCOPE_INIT)
       reusable_handles_() {
   set_vm_tag(VMTag::kIdleTagId);
   set_user_tag(UserTags::kDefaultUserTag);
@@ skipping 865 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart