Add Observatory mojo_shell hearbeat test

mojo/dart/observatory_test/main.dart

Index: mojo/dart/observatory_test/main.dart
diff --git a/sky/engine/core/painting/DrawLooper.idl b/mojo/dart/observatory_test/main.dart
similarity index 73%
copy from sky/engine/core/painting/DrawLooper.idl
copy to mojo/dart/observatory_test/main.dart
index 8fbda9d1f82e171b595c0d03c1eb0dc652f24f23..5b480abbf8cabd78fc91aeb66be2879a6171b852 100644
--- a/sky/engine/core/painting/DrawLooper.idl
+++ b/mojo/dart/observatory_test/main.dart
@@ -2,5 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-interface DrawLooper {
-};
+import 'dart:async';
+
+Future<int> main(List args) async {

[rmacnak, 2015/06/22 20:03:37]

void main(List args) {
  // Won't exit because we don't close the shell handle.
}

[Cutch, 2015/06/22 20:15:24]

Done.

+}