Reduce CPU usage when no isolates need to be profiled.

runtime/vm/profiler.cc

Index: runtime/vm/profiler.cc
diff --git a/runtime/vm/profiler.cc b/runtime/vm/profiler.cc
index de2afea156800d813254301cd5df6486242ec88a..419c0ff05a6f936b8de5d2ba519785c7423be2d7 100644
--- a/runtime/vm/profiler.cc
+++ b/runtime/vm/profiler.cc
@@ -1585,6 +1585,7 @@ IsolateProfilerData::IsolateProfilerData(SampleBuffer* sample_buffer,
   ASSERT(sample_buffer != NULL);
   sample_buffer_ = sample_buffer;
   own_sample_buffer_ = own_sample_buffer;
+  block_count_ = 0;
 }
 
 
@@ -1597,6 +1598,23 @@ IsolateProfilerData::~IsolateProfilerData() {
 }
 
 
+void IsolateProfilerData::Block() {
+  block_count_++;
+}
+
+
+void IsolateProfilerData::Unblock() {
+  block_count_--;
+  if (block_count_ < 0) {
+    FATAL("Too many calls to Dart_IsolateUnblocked.");
+  }
+  if (!blocked()) {
+    // We just unblocked this isolate, wake up the thread interrupter.
+    ThreadInterrupter::WakeUp();
+  }
+}
+
+
 Sample* SampleBuffer::ReserveSample() {
   ASSERT(samples_ != NULL);
   uintptr_t cursor = AtomicOperations::FetchAndIncrement(&cursor_);