Add property base to the Uri class

tests/corelib/uri_base_test.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.
-// This test is making sure we don't try to constant fold numbers
-// where the semantics differ at compile-time (Dart) and runtime (JS).
 
 import "package:expect/expect.dart";
 
-foo() => 0.0;
-bar() => 0;
-
 main() {
-  Expect.equals(foo(), bar());
-  Expect.equals(0.0, 0);
+  try {
+    Uri base = Uri.base;
+    Expect.isTrue(Uri.base.scheme == "file" || Uri.base.scheme == "http");
+  } on UnsupportedError catch (e) {
+    Expect.isTrue(
+        e.toString().contains("'Uri.base' is not supported"));
+  }
 }