Try to retain original structure of URI.

tests/corelib/uri_test.dart

Index: tests/corelib/uri_test.dart
diff --git a/tests/corelib/uri_test.dart b/tests/corelib/uri_test.dart
index 703515d537341a32863c74bf89b28e3bd4c583c0..298ef12d58e31d04bef534c20ee7ab47d5711804 100644
--- a/tests/corelib/uri_test.dart
+++ b/tests/corelib/uri_test.dart
@@ -352,6 +352,17 @@ void testNormalization() {
   Expect.equals("file:///y", uri.toString());
   uri = new Uri(scheme: "file", path: "y");
   Expect.equals("file:///y", uri.toString());
+
+  // Empty host/query/fragment ensures the delimiter is there.
+  // Different from not being there.
+  Expect.equals("scheme:/", Uri.parse("scheme:/").toString());
+  Expect.equals("scheme:/",
+                new Uri(scheme: "scheme", path: "/").toString());
+
+  Expect.equals("scheme:///?#", Uri.parse("scheme:///?#").toString());
+  Expect.equals("scheme:///?#",
+                new Uri(scheme: "scheme", host: "", path: "/",
+                        query: "", fragment: "").toString());
 }
 
 main() {
@@ -370,7 +381,7 @@ main() {
                           path: "/a/b/c",
                           query: "query",
                           fragment: "fragment").toString());
-  Expect.stringEquals("//null@null/a/b/c/",
+  Expect.stringEquals("/a/b/c/",
                       new Uri(
                           scheme: null,
                           userInfo: null,