Fix type annotation in _FileBase._exceptionFromResponse

Fix type annotation in _FileBase._exceptionFromResponse

ArgumentError is not a subtype of Exception, which leads to confusing errors in checked mode:

  Expected future to complete successfully, but it failed with type 'ArgumentError' is not a subtype of type 'Exception' of 'function result'.
  Stack trace:
    #0      _FileBase._exceptionFromResponse (dart:io:1032:48)
    #1      _RandomAccessFile.writeString.<anonymous closure> (dart:io:1491:37)
    #2      _FutureImpl.transform.<anonymous closure> (bootstrap:855:37)

Committed: https://code.google.com/p/dart/source/detail?r=14243

[Jennifer Messerly, 2012-10-30 04:36:54]

Figured that sending a patch was more useful than opening a bug :)

If there's a good place for a dart:io test for this, let me know. I'm not sure
how to write one that isn't too specific to this one case (and hence unlikely to
fail in the future). Perhaps we can run dart_analyzer on this code with
fatal_type_errors option? That would prevent any error of this sort, anywhere in
dart:io code.

It does seem that running: 
dart_analyzer --show-sdk-warnings runtime/bin/io.dart

finds various potential problems.

[Mads Ager (google), 2012-10-30 05:45:09]

LGTM! Thanks. I have this exact change pending in a larger changelist to use
patching for dart:io but I might not be able to land that today.