Port dart:io Path tests to package:path.

pkg/path/README.md

 A comprehensive, cross-platform path manipulation library for Dart.
 
 The path package provides common operations for manipulating file paths:
 joining, splitting, normalizing, etc.
 
 We've tried very hard to make this library do the "right" thing on whatever
 platform you run it on. When you use the top-level functions, it will assume the
 current platform's path style and work with that. If you want to specifically
 work with paths of a specific style, you can construct a `path.Builder` for that
 style.
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 Gets the part of [path] before the last separator.
 
     path.dirname('path/to/foo.dart'); // -> 'path/to'
     path.dirname('path/to');          // -> 'to'
 
 Trailing separators are ignored.
 
     builder.dirname('path/to/'); // -> 'path'
 
+If an absolute path contains no directories, only a root, then the root
+is returned.
+
+    path.dirname('/');    // -> '/' (posix)
+    path.dirname('c:\');  // -> 'c:\' (windows)
+
+If a relative path has no directories, then '.' is returned.
+    path.dirname('foo');  // -> '.'
+    path.dirname('');     // -> '.'
+
 ### String extension(String path)
 
 Gets the file extension of [path]: the portion of [basename] from the last
 `.` to the end (including the `.` itself).
 
     path.extension('path/to/foo.dart');    // -> '.dart'
     path.extension('path/to/foo');         // -> ''
     path.extension('path.to/foo');         // -> ''
     path.extension('path/to/foo.dart.js'); // -> '.js'
 
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     path.relative('/root/path/a/b.dart'); // -> 'a/b.dart'
     path.relative('/root/other.dart'); // -> '../other.dart'
 
 If the [from] argument is passed, [path] is made relative to that instead.
 
     path.relative('/root/path/a/b.dart',
         from: '/root/path'); // -> 'a/b.dart'
     path.relative('/root/other.dart',
         from: '/root/path'); // -> '../other.dart'
 
+If [path] and/or [from] are relative paths, they are assumed to be relative
+to the current directory.
+
 Since there is no relative path from one drive letter to another on Windows,
 this will return an absolute path in that case.
 
     // Windows
     path.relative(r'D:\other', from: r'C:\home'); // -> 'D:\other'
 
     // URL
     path.relative('http://dartlang.org', from: 'http://pub.dartlang.org');
       // -> 'http://dartlang.org'
 
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  *  It can accurately tell if a path is absolute based on drive-letters or UNC
     prefix.
 
  *  It understands that "/foo" is not an absolute path on Windows.
 
  *  It knows that "C:\foo\one.txt" and "c:/foo\two.txt" are two files in the
     same directory.
 
 If you find a problem, surprise or something that's unclear, please don't
 hesitate to [file a bug](http://dartbug.com/new) and let us know.