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

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 37183)
+++ runtime/vm/isolate.h	(working copy)
@@ -15,6 +15,7 @@
 #include "vm/random.h"
 #include "vm/store_buffer.h"
 #include "vm/tags.h"
+#include "vm/trace_buffer.h"
 #include "vm/timer.h"
 
 namespace dart {
@@ -446,6 +447,13 @@
     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);
@@ -597,6 +605,9 @@
   // 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_;