Fix dart warnings.

runtime/observatory/lib/src/elements/general_error.dart

 // Copyright (c) 2013, 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 general_error_element;
 
 import 'observatory_element.dart';
 import 'package:polymer/polymer.dart';
-import 'package:observatory/service.dart';
 
 /// Displays an error message
 @CustomTag('general-error')
 class GeneralErrorElement extends ObservatoryElement {
   @published String message;
 
   GeneralErrorElement.created() : super.created();
 }