Add Folder.getChildAssumingFile().

pkg/analyzer/lib/file_system/physical_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.physical_file_system;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 import 'dart:io' as io;
 
@@ skipping 161 matching lines @@
     return absolutePathContext.isWithin(this.path, path);
   }
 
   @override
   Resource getChild(String relPath) {
     String canonicalPath = canonicalizePath(relPath);
     return PhysicalResourceProvider.INSTANCE.getResource(canonicalPath);
   }
 
   @override
+  _PhysicalFile getChildAssumingFile(String relPath) {
+    String canonicalPath = canonicalizePath(relPath);
+    io.File file = new io.File(canonicalPath);
+    return new _PhysicalFile(file);
+  }
+
+  @override
   _PhysicalFolder getChildAssumingFolder(String relPath) {
     String canonicalPath = canonicalizePath(relPath);
     io.Directory directory = new io.Directory(canonicalPath);
     return new _PhysicalFolder(directory);
   }
 
   @override
   List<Resource> getChildren() {
     try {
       List<Resource> children = <Resource>[];
@@ skipping 59 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }