Add toUri and fromUri functions to pathos.

pkg/pathos/README.md

 A comprehensive, cross-platform path manipulation library for Dart.
 
 The pathos library 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.
@@ skipping 198 matching lines @@
     // URL
     path.relative('http://dartlang.org', from: 'http://pub.dartlang.org');
       // -> 'http://dartlang.org'
 
 ### String withoutExtension(String path)
 
 Removes a trailing extension from the last part of [path].
 
     withoutExtension('path/to/foo.dart'); // -> 'path/to/foo'
 
+### String fromUri(Uri uri)
+
+Returns the path represented by [uri]. For POSIX and Windows styles, [uri] must
+be a `file:` URI. For the URL style, this will just convert [uri] to a string.
+
+    // POSIX
+    path.fromUri(Uri.parse('file:///path/to/foo'))
+      // -> '/path/to/foo'
+
+    // Windows
+    path.fromUri(Uri.parse('file:///C:/path/to/foo'))
+      // -> r'C:\path\to\foo'
+
+    // URL
+    path.fromUri(Uri.parse('http://dartlang.org/path/to/foo'))
+      // -> 'http://dartlang.org/path/to/foo'
+
+### Uri toUri(String path)
+
+Returns the URI that represents [path]. For POSIX and Windows styles, this will
+return a `file:` URI. For the URL style, this will just convert [path] to a
+[Uri].
+
+This will always convert relative paths to absolute ones before converting
+to a URI.
+
+    // POSIX
+    path.toUri('/path/to/foo')
+      // -> Uri.parse('file:///path/to/foo')
+
+    // Windows
+    path.toUri(r'C:\path\to\foo')
+      // -> Uri.parse('file:///C:/path/to/foo')
+
+    // URL
+    path.toUri('http://dartlang.org/path/to/foo')
+      // -> Uri.parse('http://dartlang.org/path/to/foo')
+
 ## The path.Builder class
 
 In addition to the functions, path exposes a `path.Builder` class. This lets
 you configure the root directory and path style that paths are built using
 explicitly instead of assuming the current working directory and host OS's path
 style.
 
 You won't often use this, but it can be useful if you do a lot of path
 manipulation relative to some root directory.
 
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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.