Fix for opening SDK sources.

pkg/analyzer/lib/file_system/memory_file_system.dart

 // Copyright (c) 2014, 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 memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
@@ skipping 238 matching lines @@
     return false;
   }
 
   @override
   bool exists() => _file.exists;
 
   @override
   Uri resolveRelativeUri(Uri relativeUri) {
     return uri.resolveUri(relativeUri);
   }
+
+  @override
+  String toString() => _file.toString();
 }
 
 
 /**
  * An in-memory implementation of [Folder].
  */
 class _MemoryFolder extends _MemoryResource implements Folder {
   _MemoryFolder(MemoryResourceProvider provider, String path) :
       super(provider, path);
   @override
@@ skipping 74 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }