Fix Path.relativeTo and add support for relative paths as input.

sdk/lib/io/path.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 /**
  * A Path is an immutable wrapper of a String, with additional member functions
  * for useful path manipulations and queries.
- * On the Windows platform, Path also converts from and to native paths.
+ * On the Windows platform, Path also converts from native paths to paths using
+ * '/' as a path separator, and vice versa.
  *
  * Joining of paths and path normalization handle '.' and '..' in the usual way.
  */
 abstract class Path {
   /**
    * Creates a Path from a String that uses the native filesystem's conventions.
    *
    * On Windows, this converts '\' to '/' and has special handling for drive
    * letters and shares.
    *
    * If the path starts with a drive letter, like 'C:',  a '/' is added
    * before the drive letter.
    *
    *     new Path(r'c:\a\b').toString() == '/c:/a/b'
    *
-   * A path starting with '/c:/' (or any other character instead of 'c') is
+   * A path starting with a drive letter is
    * treated specially.  Backwards links ('..') cannot cancel the drive letter.
    *
    * If the path is a share path this is recorded in the Path object and
    * maintained in operations on the Path object.
    *
    *     var share = new Path(r'\\share\a\b\c');
    *     share.isWindowsShare == true
    *     share.toString() == '/share/a/b/c'
    *     share.toNativePath() == r'\\share\a\b\c'
    *     share.append('final').isWindowsShare == true
    */
   factory Path(String source) => new _Path(source);
 
   /**
    * Creates a Path from the String [source].  [source] is used as-is, so if
    * the string does not consist of segments separated by forward slashes, the
    * behavior may not be as expected.  Paths are immutable.
    */
   factory Path.raw(String source) => new _Path.raw(source);
 
   /**
    * Is this path the empty string?
    */
   bool get isEmpty;
 
   /**
-   * Is this path an absolute path, beginning with a path separator?
+   * Is this path an absolute path, beginning with a '/'?  Note that
+   * Windows paths beginning with '\' or with a drive letter are absolute,
+   * and a leading '/' is added when they are converted to a Path.
    */
   bool get isAbsolute;
 
   /**
    * Is this path a Windows share path?
    */
   bool get isWindowsShare;
 
   /**
-   * Does this path end with a path separator?
+   * Does this path end with a '/'?
    */
   bool get hasTrailingSeparator;
 
   /**
-   * Does this path contain no consecutive path separators, no segments that
+   * Does this path contain no consecutive '/'s, no segments that
    * are '.' unless the path is exactly '.', and segments that are '..' only
    * as the leading segments on a relative path?
    */
   bool get isCanonical;
 
   /**
    * Make a path canonical by dropping segments that are '.', cancelling
    * segments that are '..' with preceding segments, if possible,
-   * and combining consecutive path separators.  Leading '..' segments
+   * and combining consecutive '/'s.  Leading '..' segments
    * are kept on relative paths, and dropped from absolute paths.
    */
   Path canonicalize();
 
   /**
    * Joins the relative path [further] to this path.  Canonicalizes the
    * resulting joined path using [canonicalize],
    * interpreting '.' and '..' as directory traversal commands, and removing
-   * consecutive path separators.
+   * consecutive '/'s.
    *
    * If [further] is an absolute path, an IllegalArgument exception is thrown.
    *
    * Examples:
    *   `new Path('/a/b/c').join(new Path('d/e'))` returns the Path object
    *   containing `'a/b/c/d/e'`.
    *
    *   `new Path('a/b/../c/').join(new Path('d/./e//')` returns the Path
    *   containing `'a/c/d/e/'`.
    *
    *   `new Path('a/b/c').join(new Path('d/../../e')` returns the Path
    *   containing `'a/b/e'`.
    *
    * Note that the join operation does not drop the last segment of the
-   * base path, the way URL joining does.  That would be accomplished with
-   * basepath.directoryPath.join(further).
+   * base path, the way URL joining does.  To join basepath to further using
+   * URL semantics, use
+   *    [:basepath.directoryPath.join(further):].
    *
    * If you want to avoid joins that traverse
    * parent directories in the base, you can check whether
    * `further.canonicalize()` starts with '../' or equals '..'.
    */
   Path join(Path further);
 
 
   /**
    * Returns a path [:relative:] such that
    *    [:base.join(relative) == this.canonicalize():].
    * Throws an exception if such a path is impossible.
    * For example, if [base] is '../../a/b' and [this] is '.'.
    * The computation is independent of the file system and current directory.
+   *
+   * To compute a relative path using URL semantics, where the final
+   * path component of the base is dropped unless it ends with a slash,
+   * call [: a.relativeTo(b.directoryPath) :] instead of [: a.relativeTo(b) :].
    */
   Path relativeTo(Path base);
 
   /**
    * Converts a path to a string using the native filesystem's conventions.
    *
-   * On Windows, converts path separators to backwards slashes, and removes
-   * the leading path separator if the path starts with a drive specification.
+   * On Windows, converts '/'s to backwards slashes, and removes
+   * the leading '/' if the path starts with a drive specification.
    * For most valid Windows paths, this should be the inverse of the
-   * constructor Path.fromNative.
+   * conversion that the constructor new Path() performs.  If the path is
+   * a Windows share, restores the '\\' at the start of the path.
    */
   String toNativePath();
 
   /**
    * Returns the path as a string.  If this path is constructed using
    * new Path.raw(), or new Path() on a non-Windows system, the
    * returned value is the original string argument to the constructor.
    */
   String toString();
 
   /**
-   * Gets the segments of a Path.  Paths beginning or ending with the
-   * path separator do not have leading or terminating empty segments.
-   * Other than that, the segments are just the result of splitting the
-   * path on the path separator.
+   * Gets the segments of a Path. The segments are just the result of
+   * splitting the path on any '/' characters, except that a '/' at the
+   * beginning does not create an empty segment before it, and a '/' at
+   * the end does not create an empty segment after it.
    *
    *     new Path('/a/b/c/d').segments() == ['a', 'b', 'c', d'];
    *     new Path(' foo bar //../') == [' foo bar ', '', '..'];
    */
   List<String> segments();
 
   /**
-   * Appends [finalSegment] to a path as a new segment.  Adds a path separator
+   * Appends [finalSegment] to a path as a new segment.  Adds a '/'
    * between the path and [finalSegment] if the path does not already end in
-   * a path separator.  The path is not canonicalized, and [finalSegment] may
-   * contain path separators.
+   * a '/'.  The path is not canonicalized, and [finalSegment] may
+   * contain '/'s.
    */
   Path append(String finalSegment);
 
   /**
-   * Drops the final path separator and whatever follows it from this Path,
-   * and returns the resulting Path object.  If the only path separator in
+   * Drops the final '/' and whatever follows it from this Path,
+   * and returns the resulting Path object.  If the only '/' in
    * this Path is the first character, returns '/' instead of the empty string.
-   * If there is no path separator in the Path, returns the empty string.
+   * If there is no '/' in the Path, returns the empty string.
    *
    *     new Path('../images/dot.gif').directoryPath == '../images'
    *     new Path('/usr/geoffrey/www/').directoryPath == '/usr/geoffrey/www'
    *     new Path('lost_file_old').directoryPath == ''
    *     new Path('/src').directoryPath == '/'
    *     Note: new Path('/D:/src').directoryPath == '/D:'
    */
   Path get directoryPath;
 
   /**
-   * The part of the path after the last path separator, or the entire path if
-   * it contains no path separator.
+   * The part of the path after the last '/', or the entire path if
+   * it contains no '/'.
    *
    *     new Path('images/DSC_0027.jpg).filename == 'DSC_0027.jpg'
    *     new Path('users/fred/').filename == ''
    */
   String get filename;
 
   /**
    * The part of [filename] before the last '.', or the entire filename if it
    * contains no '.'.  If [filename] is '.' or '..' it is unchanged.
    *
    *     new Path('/c:/My Documents/Heidi.txt').filenameWithoutExtension
    *     would return 'Heidi'.
    *     new Path('not what I would call a path').filenameWithoutExtension
    *     would return 'not what I would call a path'.
    */
   String get filenameWithoutExtension;
 
   /**
    * The part of [filename] after the last '.', or '' if [filename]
    * contains no '.'.  If [filename] is '.' or '..', returns ''.
    *
    *     new Path('tiger.svg').extension == 'svg'
    *     new Path('/src/dart/dart_secrets').extension == ''
    */
   String get extension;
 }