Add new Dart API call Dart_MakeIsolateRunnable(). This would allow an

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 20945)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -938,6 +938,18 @@
 }
 
 
+DART_EXPORT Dart_Handle Dart_MakeIsolateRunnable() {
+  Isolate* isolate = Isolate::Current();
+  CHECK_ISOLATE(isolate);
+  bool retval = isolate->MakeRunnable();
+  if (retval == false) {
+    return Api::NewError("%s expects the isolate to not be already runnable.",
+                         CURRENT_FUNC);
+  }
+  return Api::Success(isolate);
+}
+
+
 // --- Messages and Ports ---
 
 DART_EXPORT void Dart_SetMessageNotifyCallback(