Reduce CPU usage when no isolates need to be profiled.

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 4c04cba04ea5fe673abe48b4a715201a6fefb073..dfcb94cf479630bbd937d5d1a7d9f6017f9a0e7f 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -25,6 +25,7 @@
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/port.h"
+#include "vm/profiler.h"
 #include "vm/resolver.h"
 #include "vm/reusable_handles.h"
 #include "vm/service.h"
@@ -1313,6 +1314,28 @@ DART_EXPORT void Dart_EnterIsolate(Dart_Isolate isolate) {
 }
 
 
+DART_EXPORT void Dart_IsolateBlocked() {
+  Isolate* isolate = Isolate::Current();
+  CHECK_ISOLATE(isolate);
+  IsolateProfilerData* profiler_data = isolate->profiler_data();
+  if (profiler_data == NULL) {
+    return;
+  }
+  profiler_data->Block();
+}
+
+
+DART_EXPORT void Dart_IsolateUnblocked() {
+  Isolate* isolate = Isolate::Current();
+  CHECK_ISOLATE(isolate);
+  IsolateProfilerData* profiler_data = isolate->profiler_data();
+  if (profiler_data == NULL) {
+    return;
+  }
+  profiler_data->Unblock();
+}
+
+
 DART_EXPORT void Dart_ExitIsolate() {
   CHECK_ISOLATE(Isolate::Current());
   Isolate::SetCurrent(NULL);