Reduce CPU usage when no isolates need to be profiled.

runtime/bin/dartutils.h

Index: runtime/bin/dartutils.h
diff --git a/runtime/bin/dartutils.h b/runtime/bin/dartutils.h
index 4175eb560803c1b731fce930c7a37d9b40467297..ed53fab2ef5523b7546eaf53ff14e22f692a4686 100644
--- a/runtime/bin/dartutils.h
+++ b/runtime/bin/dartutils.h
@@ -542,6 +542,18 @@ class CObjectExternalUint8Array : public CObject {
   DISALLOW_COPY_AND_ASSIGN(CObjectExternalUint8Array);
 };
 
+
+class ScopedBlockingCall {
+ public:
+  ScopedBlockingCall() {
+    Dart_IsolateBlocked();
+  }
+
+  ~ScopedBlockingCall() {
+    Dart_IsolateUnblocked();
+  }
+};
+
 }  // namespace bin
 }  // namespace dart