Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/lib/src/internal_style.dart

Index: pkg/path/lib/src/internal_style.dart
diff --git a/pkg/path/lib/src/internal_style.dart b/pkg/path/lib/src/internal_style.dart
index 67b5d343eb748a40f868c4536df7d27b3262b6df..3a95745b87a9c383b1fea9729fbfc30a53cb86d5 100644
--- a/pkg/path/lib/src/internal_style.dart
+++ b/pkg/path/lib/src/internal_style.dart
@@ -33,6 +33,9 @@ abstract class InternalStyle extends Style {
   /// "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);