Add an intrinsified early out path for Dart timeline calls

runtime/vm/timeline.h

 // Copyright (c) 2015, 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_TIMELINE_H_
 #define VM_TIMELINE_H_
 
 #include "include/dart_tools_api.h"
 
 #include "vm/allocation.h"
@@ skipping 53 matching lines @@
   // NOTE: It is not allowed to call StartEvent again without completing
   // the first event.
   TimelineEvent* StartEvent();
 
   static intptr_t enabled_offset() {
     return OFFSET_OF(TimelineStream, enabled_);
   }
 
  private:
   const char* name_;
+
+  // This field is accessed by generated code (intrinsic) and expects to see
+  // 0 or 1. If this becomes a BitField, the generated code must be updated.
   uintptr_t enabled_;
 };
 
 class Timeline : public AllStatic {
  public:
   // Initialize timeline system. Not thread safe.
   static void InitOnce();
 
   // Shutdown timeline system. Not thread safe.
   static void Shutdown();
@@ skipping 794 matching lines @@
 
  private:
   TimelineEventBlock* current_;
   TimelineEventRecorder* recorder_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_TIMELINE_H_