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

test/relative_test.dart

Index: test/relative_test.dart
diff --git a/test/relative_test.dart b/test/relative_test.dart
index 4523c8d77dfdac0a1756c7384e31d31bac64b110..57a90463da5553badc8a9401b095b8f26823d79f 100644
--- a/test/relative_test.dart
+++ b/test/relative_test.dart
@@ -31,7 +31,7 @@ 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');