Cleanup VM service

runtime/bin/vmservice/running_isolates.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.
 
 part of vmservice;
 
 class RunningIsolates implements ServiceRequestRouter {
   final Map<int, RunningIsolate> isolates = new Map<int, RunningIsolate>();
 
   RunningIsolates();
@@ skipping 22 matching lines @@
         'name': runningIsolate.name
         });
     });
     result['type'] = 'IsolateList';
     result['members'] = members;
     request.setResponse(JSON.encode(result));
   }
 
   Future route(ServiceRequest request) {
     if (request.pathSegments.length == 0) {
-      return null;
+      request.setErrorResponse('No path.');
+      return new Future.value(request);
     }
     if (request.pathSegments[0] != 'isolates') {
-      return null;
+      request.setErrorResponse('Path must begin with /isolates/.');
+      return new Future.value(request);
     }
     if (request.pathSegments.length == 1) {
       // Requesting list of running isolates.
       _isolateCollectionRequest(request);
       return new Future.value(request);
     }
     var isolateId;
     try {
       isolateId = int.parse(request.pathSegments[1]);
     } catch (e) {
       request.setErrorResponse('Could not parse isolate id: $e');
       return new Future.value(request);
     }
     var isolate = isolates[isolateId];
     if (isolate == null) {
       request.setErrorResponse('Cannot find isolate id: $isolateId');
       return new Future.value(request);
     }
     // Consume '/isolates/isolateId'
     request.pathSegments.removeRange(0, 2);
     if (request.pathSegments.length == 0) {
       // The request is now empty.
       request.setErrorResponse('No request for isolate: /isolates/$isolateId');
       return new Future.value(request);
     }
     return isolate.route(request);
   }
 }