Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/lib/src/style/posix.dart

Index: pkg/path/lib/src/style/posix.dart
diff --git a/pkg/path/lib/src/style/posix.dart b/pkg/path/lib/src/style/posix.dart
index b8b82b406d33f31a08d5c4a05c67b86dbdbc4b2f..74aeb4c5381e2af897257934802caf34047ca499 100644
--- a/pkg/path/lib/src/style/posix.dart
+++ b/pkg/path/lib/src/style/posix.dart
@@ -30,11 +30,13 @@ class PosixStyle extends InternalStyle {
   bool needsSeparator(String path) =>
       path.isNotEmpty && !isSeparator(path.codeUnitAt(path.length - 1));
 
-  String getRoot(String path) {
-    if (path.isNotEmpty && isSeparator(path.codeUnitAt(0))) return '/';
-    return null;
+  int rootLength(String path) {
+    if (path.isNotEmpty && isSeparator(path.codeUnitAt(0))) return 1;
+    return 0;
   }
 
+  bool isRootRelative(String path) => false;
+
   String getRelativeRoot(String path) => null;
 
   String pathFromUri(Uri uri) {