Expose the Observatory URL when debugging.

lib/src/runner/environment.dart

 // 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.
 
 library test.runner.environment;
 
 import '../util/cancelable_future.dart';
 
 /// The abstract class of environments in which test suites are
 /// loaded—specifically, browsers and the Dart VM.
 abstract class Environment {
+  /// The URL of the Dart VM Observatory for this environment, or `null` if this
+  /// environment doesn't run the Dart VM or the URL couldn't be detected.
+  Uri get observatoryUrl;
+
   /// Displays information indicating that the test runner is paused.
   ///
   /// The returned future will complete when the user takes action within the
   /// environment that should unpause the runner. If the runner is unpaused
   /// elsewhere, the future should be canceled.
   CancelableFuture displayPause();
 }