Refactor pkg/path.

pkg/path/README.md

 A comprehensive, cross-platform path manipulation library for Dart.
 
 The path package provides common operations for manipulating 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, including in the browser. When you use the top-level
 functions, it will assume the current platform's path style and work with
 that. If you want to explicitly work with paths of a specific style, you can
-construct a `path.Builder` for that style.
+construct a `path.Context` for that style.
 
 ## Using
 
 The path library was designed to be imported with a prefix, though you don't
 have to if you don't want to:
 
     import 'package:path/path.dart' as path;
 
 The most common way to use the library is through the top-level functions.
 These manipulate path strings based on your current working directory and
 the path style (POSIX, Windows, or URLs) of the host platform. For example:
 
     path.join("directory", "file.txt");
 
 This calls the top-level [join] function to join "directory" and
 "file.txt" using the current platform's directory separator.
 
 If you want to work with paths for a specific platform regardless of the
 underlying platform that the program is running on, you can create a
-[Builder] and give it an explicit [Style]:
+[Context] and give it an explicit [Style]:
 
-    var builder = new path.Builder(style: Style.windows);
-    builder.join("directory", "file.txt");
+    var context = new path.Context(style: Style.windows);
+    context.join("directory", "file.txt");
 
 This will join "directory" and "file.txt" using the Windows path separator,
 even when the program is run on a POSIX machine.
 
 ## FAQ
 
 ### Where can I use this?
 
 Pathos runs on the Dart VM and in the browser under both dart2js and Dartium.
 Under dart2js, it currently returns "." as the current working directory, while
@@ skipping 34 matching lines @@
 
  *  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.
 
 ### What is a "path" in the browser?
 
 If you use this package in a browser, then it considers the "platform" to be
 the browser itself and uses URL strings to represent "browser paths".