Reduce CPU usage when no isolates need to be profiled.

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"
@@ skipping 469 matching lines @@
 
   void set_thread_state(InterruptableThreadState* state) {
     ASSERT((thread_state_ == NULL) || (state == NULL));
     thread_state_ = state;
   }
 
   InterruptableThreadState* thread_state() const {
     return thread_state_;
   }
 
-  void ProfileInterrupt();
+  // Returns the number of sampled threads.
+  intptr_t ProfileInterrupt();
 
   VMTagCounters* vm_tag_counters() {
     return &vm_tag_counters_;
   }
 
   uword user_tag() const {
     return user_tag_;
   }
   static intptr_t user_tag_offset() {
     return OFFSET_OF(Isolate, user_tag_);
@@ skipping 235 matching lines @@
   char* script_url_;
   char* library_url_;
   char* class_name_;
   char* function_name_;
   char* exception_callback_name_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_