Expose the Observatory URL when debugging.

lib/src/runner/vm/environment.dart

Index: lib/src/runner/vm/environment.dart
diff --git a/lib/src/runner/vm/environment.dart b/lib/src/runner/vm/environment.dart
index bbe341817fa1c797185483c3ac6897f3e2a18efb..984d14c17478b05dd2a60f4a8d1a49b3561c10f5 100644
--- a/lib/src/runner/vm/environment.dart
+++ b/lib/src/runner/vm/environment.dart
@@ -11,6 +11,9 @@ import '../../util/cancelable_future.dart';
 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.");