Add a README for path.

pkg/path/README.md

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+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 host OS'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.
+
+## 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:pathos/path.dart' as path;
+
+## Top-level functions
+
+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 or Windows) of the host operating system.
+
+### String get current
+
+Gets the path to the current working directory.
+
+### String get separator
+
+Gets the path separator for the current platform. On Mac and Linux, this
+is `/`. On Windows, it's `\`.
+
+### String absolute(String path)
+
+Converts [path] to an absolute path by resolving it relative to the current
+working directory. If [path] is already an absolute path, just returns it.
+
+    path.absolute('foo/bar.txt'); // -> /your/current/dir/foo/bar.txt
+
+### String basename(String path)
+
+Gets the part of [path] after the last separator.
+
+    path.basename('path/to/foo.dart'); // -> 'foo.dart'
+    path.basename('path/to');          // -> 'to'
+
+Trailing separators are ignored.
+
+    builder.basename('path/to/'); // -> 'to'
+
+### String basenameWithoutExtension(String path)
+
+Gets the part of [path] after the last separator, and without any trailing
+file extension.
+
+    path.basenameWithoutExtension('path/to/foo.dart'); // -> 'foo'
+
+Trailing separators are ignored.
+
+    builder.basenameWithoutExtension('path/to/foo.dart/'); // -> 'foo'
+
+### String dirname(String path)
+
+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'
+
+### 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'
+
+If the file name starts with a `.`, then that is not considered the
+extension:
+
+    path.extension('~/.bashrc');    // -> ''
+    path.extension('~/.notes.txt'); // -> '.txt'
+
+### String rootPrefix(String path)
+
+Returns the root of [path], if it's absolute, or the empty string if it's
+relative.
+
+    // Unix
+    path.rootPrefix('path/to/foo'); // -> ''
+    path.rootPrefix('/path/to/foo'); // -> '/'
+
+    // Windows
+    path.rootPrefix(r'path\to\foo'); // -> ''
+    path.rootPrefix(r'C:\path\to\foo'); // -> r'C:\'
+
+### bool isAbsolute(String path)
+
+Returns `true` if [path] is an absolute path and `false` if it is a
+relative path. On POSIX systems, absolute paths start with a `/` (forward
+slash). On Windows, an absolute path starts with `\\`, or a drive letter
+followed by `:/` or `:\`.
+
+### bool isRelative(String path)
+
+Returns `true` if [path] is a relative path and `false` if it is absolute.
+On POSIX systems, absolute paths start with a `/` (forward slash). On
+Windows, an absolute path starts with `\\`, or a drive letter followed by
+`:/` or `:\`.
+
+### String join(String part1, [String part2, String part3, ...])
+
+Joins the given path parts into a single path using the current platform's
+[separator]. Example:
+
+    path.join('path', 'to', 'foo'); // -> 'path/to/foo'
+
+If any part ends in a path separator, then a redundant separator will not
+be added:
+
+    path.join('path/', 'to', 'foo'); // -> 'path/to/foo
+
+If a part is an absolute path, then anything before that will be ignored:
+
+    path.join('path', '/to', 'foo'); // -> '/to/foo'
+
+### List<String> split(String path)
+
+Splits [path] into its components using the current platform's [separator].
+
+    path.split('path/to/foo'); // -> ['path', 'to', 'foo']
+
+The path will *not* be normalized before splitting.
+
+    path.split('path/../foo'); // -> ['path', '..', 'foo']
+
+If [path] is absolute, the root directory will be the first element in the
+array. Example:
+
+    // Unix
+    path.split('/path/to/foo'); // -> ['/', 'path', 'to', 'foo']
+
+    // Windows
+    path.split(r'C:\path\to\foo'); // -> [r'C:\', 'path', 'to', 'foo']
+
+### String normalize(String path)
+
+Normalizes [path], simplifying it by handling `..`, and `.`, and
+removing redundant path separators whenever possible.
+
+    path.normalize('path/./to/..//file.text'); // -> 'path/file.txt'
+String normalize(String path) => _builder.normalize(path);
+
+### String relative(String path, {String from})
+
+Attempts to convert [path] to an equivalent relative path from the current
+directory.
+
+    // Given current directory is /root/path:
+    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'
+
+Since there is no relative path from one drive letter to another on Windows,
+this will return an absolute path in that case.
+
+    path.relative(r'D:\other', from: r'C:\home'); // -> 'D:\other'
+
+### String withoutExtension(String path)
+
+Removes a trailing extension from the last part of [path].
+
+    withoutExtension('path/to/foo.dart'); // -> '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.
+
+    var builder = new path.Builder(root: '/other/root');
+    builder.relative('/other/root/foo.txt'); // -> 'foo.txt'
+
+It exposes the same methods and getters as the top-level functions, with the
+addition of:
+
+### new Builder({Style style, String root})
+
+Creates a new path builder for the given style and root directory.
+
+If [style] is omitted, it uses the host operating system's path style. If
+[root] is omitted, it defaults to the current working directory. If [root]
+is relative, it is considered relative to the current working directory.
+
+### Style style
+
+The style of path that this builder works with.
+
+### String root
+
+The root directory that relative paths will be relative to.
+
+### String get separator
+
+Gets the path separator for the builder's [style]. On Mac and Linux,
+this is `/`. On Windows, it's `\`.
+
+### String rootPrefix(String path)
+
+Returns the root of [path], if it's absolute, or an empty string if it's
+relative.
+
+    // Unix
+    builder.rootPrefix('path/to/foo'); // -> ''
+    builder.rootPrefix('/path/to/foo'); // -> '/'
+
+    // Windows
+    builder.rootPrefix(r'path\to\foo'); // -> ''
+    builder.rootPrefix(r'C:\path\to\foo'); // -> r'C:\'
+
+### String resolve(String part1, [String part2, String part3, ...])
+
+Creates a new path by appending the given path parts to the [root].
+Equivalent to [join()] with [root] as the first argument. Example:
+
+    var builder = new Builder(root: 'root');
+    builder.resolve('path', 'to', 'foo'); // -> 'root/path/to/foo'
+
+## The path.Style class
+
+The path library can work with two different "flavors" of path: POSIX and
+Windows. The differences between these are encapsulated by the `path.Style`
+enum class. There are two instances of it:
+
+### path.Style.posix
+
+POSIX-style paths use "/" (forward slash) as separators. Absolute paths
+start with "/". Used by UNIX, Linux, Mac OS X, and others.
+
+### path.Style.windows
+
+Windows paths use "\" (backslash) as separators. Absolute paths start with
+a drive letter followed by a colon (example, "C:") or two backslashes
+("\\") for UNC paths.
+
+## FAQ
+
+### Where can I use this?
+
+Currently, Dart has no way of encapsulating configuration-specific code.
+Ideally, this library would be able to import dart:io when that's available or
+dart:html when that is. That would let it seamlessly work on both.
+
+Until then, this only works on the standalone VM. It's API is not coupled to
+dart:io, but it uses it internally to determine the current working directory.
+
+### Why doesn't this make paths first-class objects?
+
+When you have path *objects*, then every API that takes a path has to decide if
+it accepts strings, path objects, or both.
+
+ *  Accepting strings is the most convenient, but then it seems weird to have
+    these path objects that aren't actually accepted by anything that needs a
+    path. Once you've created a path, you have to always call `.toString()` on
+    it before you can do anything useful with it.
+
+ *  Requiring objects forces users to wrap path strings in these objects, which
+    is tedious. It also means coupling that API to whatever library defines this
+    path class. If there are multiple "path" libraries that each define their
+    own path types, then any library that works with paths has to pick which one
+    it uses.
+
+ *  Taking both means you can't type your API. That defeats the purpose of
+    having a path type: why have a type if your APIs can't annotate that they
+    use it?
+
+Given that, we've decided this library should simply treat paths as strings.
+
+### How cross-platform is this?
+
+We believe this library handles most of the corner cases of Windows paths
+(POSIX paths are generally pretty straightforward):
+
+ *  It understands that *both* "/" and "\" are valid path separators, not just
+    "\".
+
+ *  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.