Expose the Observatory URL when debugging.

lib/src/runner/vm/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.vm.environment;
 
 import '../environment.dart';
 import '../../util/cancelable_future.dart';
 
 /// The environment for test suites loaded in an isolate in this Dart VM.
 class VMEnvironment implements Environment {
   const VMEnvironment();
 
+  Uri get observatoryUrl => throw new UnsupportedError(
+      "VMEnvironment.observatoryUrl is not currently supported.");
+
   CancelableFuture displayPause() =>
       throw new UnsupportedError(
           "The VM doesn't yet support Environment.displayPause.");
 }