Test that we can load Observatory frontend resources

runtime/observatory/tests/service/observatory_assets_test.dart

Index: runtime/observatory/tests/service/observatory_assets_test.dart
diff --git a/runtime/observatory/tests/service/observatory_assets_test.dart b/runtime/observatory/tests/service/observatory_assets_test.dart
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+++ b/runtime/observatory/tests/service/observatory_assets_test.dart
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+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+
+import 'test_helper.dart';
+
+var tests = [
+  (VM vm) async {
+    // Simple heartbeat test to ensure we can fetch Observatory resources.
+    var heartBeatUrl =
+        serviceHttpAddress + '/third_party/trace_viewer_full.html';
+    print('Trying to fetch $heartBeatUrl');
+    HttpClient client = new HttpClient();
+    HttpClientRequest request = await client.getUrl(Uri.parse(heartBeatUrl));
+    HttpClientResponse response = await request.close();
+    expect(response.statusCode, 200);
+    await response.drain();
+  }
+];
+
+main(args) async => runVMTests(args, tests);