* File.cs: Changed argument names and thrown exception to better match

* File.cs: Changed argument names and thrown exception to better match
.NET. Use String.Length instead of comparison with empty string.
* DirectoryInfo.cs: Added missing deserialization ctor.
* FileTest.cs: Renamed and improved existing tests. Avoid use of
ExpectedException.
* FileSystemInfoTest.cs: No longer derive from deprecated Assertion
class. Added (de)serialization tests.

svn path=/trunk/mcs/; revision=100870