Reduce CPU usage when no isolates need to be profiled.

runtime/vm/profiler.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_PROFILER_H_
 #define VM_PROFILER_H_
 
 #include "vm/allocation.h"
 #include "vm/code_observers.h"
 #include "vm/globals.h"
@@ skipping 57 matching lines @@
  public:
   IsolateProfilerData(SampleBuffer* sample_buffer, bool own_sample_buffer);
   ~IsolateProfilerData();
 
   SampleBuffer* sample_buffer() const { return sample_buffer_; }
 
   void set_sample_buffer(SampleBuffer* sample_buffer) {
     sample_buffer_ = sample_buffer;
   }
 
+  bool blocked() const {
+    return block_count_ > 0;
+  }
+
+  void Block();
+
+  void Unblock();
+
  private:
   SampleBuffer* sample_buffer_;
   bool own_sample_buffer_;
+  intptr_t block_count_;
 
   DISALLOW_COPY_AND_ASSIGN(IsolateProfilerData);
 };
 
 
 class SampleVisitor : public ValueObject {
  public:
   explicit SampleVisitor(Isolate* isolate) : isolate_(isolate), visited_(0) { }
   virtual ~SampleVisitor() {}
 
@@ skipping 206 matching lines @@
   intptr_t capacity_;
   uintptr_t cursor_;
 
   DISALLOW_COPY_AND_ASSIGN(SampleBuffer);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_PROFILER_H_