Reduce CPU usage when no isolates need to be profiled.

runtime/include/dart_api.h

 /*
  * Copyright (c) 2012, 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 INCLUDE_DART_API_H_
 #define INCLUDE_DART_API_H_
 
 /** \mainpage Dart Embedding API Reference
@@ skipping 964 matching lines @@
  * the current isolate will be set to the provided isolate.
  *
  * Requires there to be no current isolate.
  */
 DART_EXPORT void Dart_EnterIsolate(Dart_Isolate isolate);
 /* TODO(turnidge): Describe what happens if two threads attempt to
  * enter the same isolate simultaneously. Check for this in the code.
  * Describe whether isolates are allowed to migrate. */
 
 /**
+ * Notifies the VM that the current isolate is about to make a blocking call.
+ */
+DART_EXPORT void Dart_IsolateBlocked();
+
+/**
+ * Notifies the VM that the current isolate is no longer blocked.
+ */
+DART_EXPORT void Dart_IsolateUnblocked();
+
+/**
  * Exits an isolate. After this call, Dart_CurrentIsolate will
  * return NULL.
  *
  * Requires there to be a current isolate.
  */
 DART_EXPORT void Dart_ExitIsolate();
 /* TODO(turnidge): We don't want users of the api to be able to exit a
  * "pure" dart isolate. Implement and document. */
 
 /**
@@ skipping 1719 matching lines @@
  * NOTE: If multiple callbacks with the same name are registered, only the
  * last callback registered will be remembered.
  */
 DART_EXPORT void Dart_RegisterRootServiceRequestCallback(
     const char* name,
     Dart_ServiceRequestCallback callback,
     void* user_data);
 
 
 #endif  /* INCLUDE_DART_API_H_ */  /* NOLINT */