More test clean-up

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 analyzer.file_system.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:core';
@@ skipping 218 matching lines @@
    */
   void writeOn(StringSink sink) {
     List<String> paths = _pathToResource.keys.toList();
     paths.sort();
     paths.forEach(sink.writeln);
   }
 
   void _checkFileAtPath(String path) {
     _MemoryResource resource = _pathToResource[path];
     if (resource is! _MemoryFile) {
+      if (resource == null) {
+        throw new ArgumentError('File expected at "$path" but does not exist');
+      }
       throw new ArgumentError(
           'File expected at "$path" but ${resource.runtimeType} found');
     }
   }
 
   void _checkFolderAtPath(String path) {
     _MemoryResource resource = _pathToResource[path];
     if (resource is! _MemoryFolder) {
       throw new ArgumentError(
           'Folder expected at "$path" but ${resource.runtimeType} found');
@@ skipping 312 matching lines @@
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 
   @override
   Uri toUri() => _provider.pathContext.toUri(path);
 }