Port dart:io Path tests to package:path.

pkg/path/README.md

Index: pkg/path/README.md
diff --git a/pkg/path/README.md b/pkg/path/README.md
index dacacaa8396c66c6027eba754b9611c1d9f8cb51..4108ce0c103bbc438cd778d66f14583f58426974 100644
--- a/pkg/path/README.md
+++ b/pkg/path/README.md
@@ -73,6 +73,16 @@ 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
@@ -200,6 +210,9 @@ If the [from] argument is passed, [path] is made relative to that instead.
     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.