Add data-URI support class to dart:core (next to Uri).

tests/lib/uri/uri_query_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.
 
 import "package:expect/expect.dart";
 
 void testInvalidArguments() {
 }
 
 void testEncodeQueryComponent() {
@@ skipping 160 matching lines @@
   test(new Uri(queryParameters: {"a": "b", "c": "d"}).queryParameters);
 }
 
 main() {
   testInvalidArguments();
   testEncodeQueryComponent();
   testQueryParameters();
   testInvalidQueryParameters();
   testQueryParametersImmutableMap();
 }