Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/lib/src/internal_style.dart

 // Copyright (c) 2014, 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.
 
 library path.internal_style;
 
 import 'context.dart';
 import 'style.dart';
 
 /// The internal interface for the [Style] type.
@@ skipping 15 matching lines @@
 
   /// Returns whether this path component needs a separator after it.
   ///
   /// Windows and POSIX styles just need separators when the previous component
   /// doesn't already end in a separator, but the URL always needs to place a
   /// separator between the root and the first component, even if the root
   /// already ends in a separator character. For example, to join "file://" and
   /// "usr", an additional "/" is needed (making "file:///usr").
   bool needsSeparator(String path);
 
+  /// Returns the offset of the root part. Returns 0 if the path is relative.

[Bob Nystrom, 2014/08/04 17:17:32]

"Returns the offset" -> "The number of characters".

[nweiz, 2014/08/04 20:11:57]

The style guide says the first sentence of doc comments should be its own
paragraph.

Also, explain what this does for relative roots.

[Anders Johnsen, 2014/08/05 08:57:13]

Done.

[Anders Johnsen, 2014/08/05 08:57:13]

Done.

+  int rootLength(String path);
+
   /// Gets the root prefix of [path] if path is absolute. If [path] is relative,
   /// returns `null`.
   String getRoot(String path);
 
   /// Gets the root prefix of [path] if it's root-relative.
   ///
   /// If [path] is relative or absolute and not root-relative, returns `null`.
   String getRelativeRoot(String path);
 
   /// Returns the path represented by [uri] in this style.
   String pathFromUri(Uri uri);
 
   /// Returns the URI that represents the relative path made of [parts].
   Uri relativePathToUri(String path) =>
       new Uri(pathSegments: context.split(path));
 
   /// Returns the URI that represents [path], which is assumed to be absolute.
   Uri absolutePathToUri(String path);
 }