Check service availability on editor repository

runtime/observatory/lib/src/repositories/editor.dart

 // Copyright (c) 2016, 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
 
 part of repositories;
 
 class EditorRepository extends M.EditorRepository {
   final S.VM _vm;
   final String _editor;
 
   bool get canOpenClass => _getService() != null;
 
   EditorRepository(S.VM vm, {String editor})
       : _vm = vm,
         _editor = editor {
     assert(_vm != null);
   }
 
   S.Service _getService() {
-    if (_vm.services.isEmpty) {
-      return null;
+    Iterable<M.Service> services =
+        _vm.services.where((s) => s.service == 'openSourceLocation');
+    if (_editor != null) {
+      services = services.where((s) => s.alias == _editor);
     }
-    if (_editor == null) {
-      return _vm.services.where((s) => s.service == 'openSourceLocation').first;
+    if (services.isNotEmpty) {
+      return services.first;
     }
-    return _vm.services
-        .where((s) => s.service == 'openSourceLocation' && s.alias == _editor)
-        .single;
+    return null;
   }
 
   Future openClass(M.IsolateRef i, M.ClassRef c) async {
     S.Class clazz = c as S.Class;
     assert(clazz != null);
     if (!clazz.loaded) {
       await clazz.load();
     }
     if (clazz.location == null) {
       return new Future.value();
     }
     return await openSourceLocation(i, clazz.location);
   }
 
   Future openSourceLocation(M.IsolateRef i, M.SourceLocation l) async {
     final isolate = i as S.Isolate;
     assert(isolate != null);
     assert(l != null);
     return await isolate.invokeRpc(_getService().method,
         {'scriptId': l.script.id, 'tokenPos': l.tokenPos});
   }
 }