Fix path to only use public interface in the implementation of methods.

runtime/bin/path_impl.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.
 
 class _Path implements Path {
   final String _path;
 
   _Path(String source) : _path = source;
   _Path.fromNative(String source) : _path = _clean(source);
 
@@ skipping 23 matching lines @@
   bool get hasTrailingSeparator => _path.endsWith('/');
 
   String toString() => _path;
 
   Path relativeTo(Path base) {
     // Throws exception if an unimplemented or impossible case is reached.
     // Returns a path "relative" such that
     //    base.join(relative) == this.canonicalize.
     // Throws an exception if no such path exists, or the case is not
     // implemented yet.
-    if (base.isAbsolute && _path.startsWith(base._path)) {
-      if (_path == base._path) return new Path('.');
+    var basePath = base.toString();
+    if (base.isAbsolute && _path.startsWith(basePath)) {
+      if (_path == basePath) return new Path('.');
       if (base.hasTrailingSeparator) {
-        return new Path(_path.substring(base._path.length));
+        return new Path(_path.substring(basePath.length));
       }
-      if (_path[base._path.length] == '/') {
-        return new Path(_path.substring(base._path.length + 1));
+      if (_path[basePath.length] == '/') {
+        return new Path(_path.substring(basePath.length + 1));
       }
     } else if (base.isAbsolute && isAbsolute) {
       List<String> baseSegments = base.canonicalize().segments();
       List<String> pathSegments = canonicalize().segments();
       int common = 0;
       int length = min(pathSegments.length, baseSegments.length);
       while (common < length && pathSegments[common] == baseSegments[common]) {
         common++;
       }
       final sb = new StringBuffer();
@@ skipping 21 matching lines @@
 
   Path join(Path further) {
     if (further.isAbsolute) {
       throw new ArgumentError(
           "Path.join called with absolute Path as argument.");
     }
     if (isEmpty) {
       return further.canonicalize();
     }
     if (hasTrailingSeparator) {
-      return new Path('$_path${further._path}').canonicalize();
+      return new Path('$_path${further}').canonicalize();
     }
-    return new Path('$_path/${further._path}').canonicalize();
+    return new Path('$_path/${further}').canonicalize();
   }
 
   // Note: The URI RFC names for these operations are normalize, resolve, and
   // relativize.
   Path canonicalize() {
     if (isCanonical) return this;
     return makeCanonical();
   }
 
   bool get isCanonical {
@@ skipping 129 matching lines @@
     if (pos < 0) return new Path('');
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     return new Path((pos > 0) ? _path.substring(0, pos) : '/');
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }