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

runtime/vm/isolate.h

 // 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.
 
 #ifndef VM_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/thread.h"
 #include "vm/base_isolate.h"
 #include "vm/class_table.h"
 #include "vm/handles.h"
 #include "vm/megamorphic_cache_table.h"
 #include "vm/random.h"
 #include "vm/store_buffer.h"
 #include "vm/tags.h"
+#include "vm/trace_buffer.h"
 #include "vm/timer.h"
 
 namespace dart {
 
 // Forward declarations.
 class AbstractType;
 class ApiState;
 class Array;
 class Class;
 class Code;
@@ skipping 411 matching lines @@
     return entropy_source_callback_;
   }
 
   void set_object_id_ring(ObjectIdRing* ring) {
     object_id_ring_ = ring;
   }
   ObjectIdRing* object_id_ring() {
     return object_id_ring_;
   }
 
+  void set_trace_buffer(TraceBuffer* buffer) {
+    trace_buffer_ = buffer;
+  }
+  TraceBuffer* trace_buffer() {
+    return trace_buffer_;
+  }
+
   DeoptContext* deopt_context() const { return deopt_context_; }
   void set_deopt_context(DeoptContext* value) {
     ASSERT(value == NULL || deopt_context_ == NULL);
     deopt_context_ = value;
   }
 
   intptr_t BlockClassFinalization() {
     ASSERT(defer_finalization_count_ >= 0);
     return defer_finalization_count_++;
   }
@@ skipping 131 matching lines @@
 
   // Status support.
   char* stacktrace_;
   intptr_t stack_frame_index_;
 
   bool cha_used_;
 
   // Ring buffer of objects assigned an id.
   ObjectIdRing* object_id_ring_;
 
+  // Trace buffer support.
+  TraceBuffer* trace_buffer_;
+
   IsolateProfilerData* profiler_data_;
   Mutex profiler_data_mutex_;
   InterruptableThreadState* thread_state_;
 
   VMTagCounters vm_tag_counters_;
   uword user_tag_;
   RawGrowableObjectArray* tag_table_;
   RawUserTag* current_tag_;
 
   // Isolate list next pointer.
@@ skipping 130 matching lines @@
   char* script_url_;
   char* library_url_;
   char* class_name_;
   char* function_name_;
   char* exception_callback_name_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_