Adding more relative path support.

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 31 matching lines @@
     // 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('.');
       if (base.hasTrailingSeparator) {
         return new Path(_path.substring(base._path.length));
       }
       if (_path[base._path.length] == '/') {
         return new Path(_path.substring(base._path.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();
+
+      for (int i = common + 1; i < baseSegments.length; i++) {
+        sb.add('../');
+      }
+      if (base.hasTrailingSeparator) {
+        sb.add('../');
+      }
+      for (int i = common; i < pathSegments.length - 1; i++) {
+        sb.add('${pathSegments[i]}/');
+      }
+      sb.add('${pathSegments.last()}');
+      if (_path.hasTrailingSeparator) {
+        sb.add('/');
+      }
+      return new Path(sb.toString());
     }
     throw new NotImplementedException(
       "Unimplemented case of Path.relativeTo(base):\n"
-      "  Only absolute paths with strict containment are handled at present.\n"
+      "  Only absolute paths are handled at present.\n"
       "  Arguments: $_path.relativeTo($base)");
   }
 
   Path join(Path further) {
     if (further.isAbsolute) {
       throw new ArgumentError(
           "Path.join called with absolute Path as argument.");
     }
     if (isEmpty) {
       return further.canonicalize();
@@ skipping 144 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);
   }
 }