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..5368495d2ba66471e8c59dbf6cf763662f862f2a 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"
@@ -5028,6 +5029,30 @@ DART_EXPORT Dart_Handle Dart_SetPeer(Dart_Handle object, void* peer) {
 }
 
 
+// --- Profiler support ---
+
+DART_EXPORT void Dart_BlockProfiler() {
+  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_UnblockProfiler() {
+  Isolate* isolate = Isolate::Current();
+  CHECK_ISOLATE(isolate);
+  IsolateProfilerData* profiler_data = isolate->profiler_data();
+  if (profiler_data == NULL) {
+    return;
+  }
+  profiler_data->Unblock();
+}
+
+
 // --- Service support ---
 
 DART_EXPORT Dart_Isolate Dart_GetServiceIsolate(void* callback_data) {