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

tests/lib/uri/uri_scheme_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() {
   Expect.throws(() => new Uri(scheme: "_"), (e) => e is FormatException);
   Expect.throws(() => new Uri(scheme: "http_s"), (e) => e is FormatException);
   Expect.throws(() => new Uri(scheme: "127.0.0.1:80"),
@@ skipping 17 matching lines @@
   test("http+ssl", "http+ssl:", "HTTP+ssl");
   test("urn", "urn:", "urn");
   test("urn", "urn:", "UrN");
   test("a123.432", "a123.432:", "a123.432");
 }
 
 main() {
   testInvalidArguments();
   testScheme();
 }