Clean up service isolate loading error check.

runtime/bin/dartutils.cc

Index: runtime/bin/dartutils.cc
diff --git a/runtime/bin/dartutils.cc b/runtime/bin/dartutils.cc
index 3c16a0ef7fc69b3844bf91f8f70b122062d01674..50ae7d067d6c8a0f9285ffcdc793b7f17b5bb076 100644
--- a/runtime/bin/dartutils.cc
+++ b/runtime/bin/dartutils.cc
@@ -684,9 +684,8 @@ Dart_Handle DartUtils::PrepareBuiltinLibrary(Dart_Handle builtin_lib,
     // Wait for the service isolate to initialize the load port.
     Dart_Port load_port = Dart_ServiceWaitForLoadPort();
     if (load_port == ILLEGAL_PORT) {
-      // TODO(rmacnak): Return an error handle here once precompilation_test
-      // works.
-      return NewDartUnsupportedError("Service did not return load port.");
+      return Dart_NewUnhandledExceptionError(
+          NewDartUnsupportedError("Service did not return load port."));
     }
     result = Builtin::SetLoadPort(load_port);
     RETURN_IF_ERROR(result);