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

test/relative_test.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.
 //
 // Test "relative" on all styles of path.Context, on all platforms.
 
 import "package:unittest/unittest.dart";
 import "package:path/path.dart" as path;
 
 import "utils.dart";
@@ skipping 13 matching lines @@
     relativeTest(
         new path.Context(style: path.Style.url, current: 'file:///'), '/');
     relativeTest(new path.Context(style: path.Style.url, current: '.'), '/');
   });
 }
 
 void relativeTest(path.Context context, String prefix) {
   var isRelative = (context.current == '.');
   // Cases where the arguments are absolute paths.
   expectRelative(result, pathArg, fromArg) {
-    expect(context.normalize(result), context.relative(pathArg, from: fromArg));
+    expect(context.relative(pathArg, from: fromArg), context.normalize(result));
   }
 
   expectRelative('c/d', '${prefix}a/b/c/d', '${prefix}a/b');
   expectRelative('c/d', '${prefix}a/b/c/d', '${prefix}a/b/');
   expectRelative('.', '${prefix}a', '${prefix}a');
   // Trailing slashes in the inputs have no effect.
   expectRelative('../../z/x/y', '${prefix}a/b/z/x/y', '${prefix}a/b/c/d/');
   expectRelative('../../z/x/y', '${prefix}a/b/z/x/y', '${prefix}a/b/c/d');
   expectRelative('../../z/x/y', '${prefix}a/b/z/x/y/', '${prefix}a/b/c/d');
   expectRelative('../../../z/x/y', '${prefix}z/x/y', '${prefix}a/b/c');
@@ skipping 33 matching lines @@
   // Should always throw - no relative path can be constructed.
   if (isRelative) {
     expect(() => context.relative('.', from: '..'), throwsPathException);
     expect(() => context.relative('a/b', from: '../../d'), throwsPathException);
     expect(() => context.relative('a/b', from: '${prefix}a/b'),
         throwsPathException);
     // An absolute path relative from a relative path returns the absolute path.
     expectRelative('${prefix}a/b', '${prefix}a/b', 'c/d');
   }
 }