--- a/lib/fakefs/file.rb
+++ b/lib/fakefs/file.rb
@@ -1,3 +1,5 @@
+require 'stringio' unless defined? StringIO
+
 module FakeFS
   class File
     PATH_SEPARATOR = '/'

The following Rakefile change extracted from https://github.com/defunkt/fakefs/commit/82db05e85d1a486fbdb6:

--- a/Rakefile
+++ b/Rakefile
@@ -1,2 +1,4 @@
+$LOAD_PATH.unshift File.join(File.dirname(__FILE__))
+
 desc "Run tests"
 task :test do

--- a/test/fakefs_test.rb
+++ b/test/fakefs_test.rb
@@ -288,7 +288,7 @@
     assert_nil File.size?("/path/to/other.txt")
   end
 
-  def test_can_check_size?_of_empty_file
+  def test_can_check_size_of_empty_file
     path = '/path/to/file.txt'
     File.open(path, 'w') do |f|
       f << ''