Don't use 'current' if 'from' is not relative.

lib/src/context.dart

 // Copyright (c) 2013, 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.context;
 
 import 'internal_style.dart';
 import 'style.dart';
 import 'parsed_path.dart';
 import 'path_exception.dart';
@@ skipping 306 matching lines @@
   /// a context with a relative path for [current].
   ///
   ///     var context = new Context(r'some/relative/path');
   ///     context.relative(r'/absolute/path'); // -> '/absolute/path'
   ///
   /// If [root] is relative, it may be impossible to determine a path from
   /// [from] to [path]. For example, if [root] and [path] are "." and [from] is
   /// "/", no path can be determined. In this case, a [PathException] will be
   /// thrown.
   String relative(String path, {String from}) {
-    from = from == null ? current : this.join(current, from);
+    if (from == null) {
+      from = current;
+    } else if (this.isRelative(from) || this.isRootRelative(from)) {

[Bob Nystrom, 2015/02/18 17:09:08]

Document this:

// Avoid calling [current] since it is slow and calling join() when [from] is
absolute does nothing.

+      from = this.join(current, from);
+    }
 
     // We can't determine the path from a relative path to an absolute path.
     if (this.isRelative(from) && this.isAbsolute(path)) {
       return this.normalize(path);
     }
 
     // If the given path is relative, resolve it relative to the context's
     // current directory.
     if (this.isRelative(path) || this.isRootRelative(path)) {
       path = this.absolute(path);
@@ skipping 212 matching lines @@
     var message = new StringBuffer();
     message.write("$method(");
     message.write(args
         .take(numArgs)
         .map((arg) => arg == null ? "null" : '"$arg"')
         .join(", "));
     message.write("): part ${i - 1} was null, but part $i was not.");
     throw new ArgumentError(message.toString());
   }
 }