Add Observatory mojo_shell hearbeat test

mojo/dart/observatory_test/main.dart

Index: mojo/dart/observatory_test/main.dart
diff --git a/mojo/public/interfaces/network/network_error.mojom b/mojo/dart/observatory_test/main.dart
similarity index 66%
copy from mojo/public/interfaces/network/network_error.mojom
copy to mojo/dart/observatory_test/main.dart
index 95b6cd17ff17463de88b92fb30ed19f1d6a89be7..12f0e05f999763c7e03561986446e5edabab8030 100644
--- a/mojo/public/interfaces/network/network_error.mojom
+++ b/mojo/dart/observatory_test/main.dart
@@ -2,11 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module mojo;
-
-struct NetworkError {
-  int32 code;
-  string? description;
-};
-
-
+void main(List args) {
+  // Won't exit because we don't close the shell handle.
+}