Remove the extra IO#dup method

Author: eregon

spec/tags/truffle/methods_tags.txt

@@ -23,7 +23,6 @@ fails:Public methods on Hash should not include sort
 fails:Public methods on IO should include fdatasync
 fails:Public methods on IO should include pread
 fails:Public methods on IO should include pwrite
-fails:Public methods on IO should not include dup
 fails:Public methods on IO should not include encode_with
 fails:Public methods on IO should not include nonblock
 fails:Public methods on IO should not include nonblock=

src/main/ruby/truffleruby/core/io.rb

@@ -1193,10 +1193,12 @@ def initialize(fd, mode=nil, options=undefined)
   ##
   # Obtains a new duplicate descriptor for the current one.
   def initialize_copy(original) # :nodoc:
+    fd = original.fileno
+
     # The 3rd argument is minimum acceptable descriptor value for the new FD.
     # We want to ensure newly allocated FDs never take the standard IO ones, even
     # if a STDIO stream is closed.
-    @descriptor = Truffle::POSIX.fcntl(@descriptor, F_DUPFD_CLOEXEC, 3)
+    @descriptor = Truffle::POSIX.fcntl(fd, F_DUPFD_CLOEXEC, 3)
   end
 
   def advise(advice, offset = 0, len = 0)
@@ -1324,11 +1326,6 @@ def closed?
     @descriptor == -1
   end
 
-  def dup
-    ensure_open
-    super
-  end
-
   # Argument matrix for IO#gets and IO#each:
   #
   #  separator / limit | nil | >= 0 | < 0