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

tests/lib/uri/uri_file_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.
 
 import "package:expect/expect.dart";
 
 testFileUri() {
   final unsupported = new UnsupportedError("");
 
   var tests = [
@@ skipping 305 matching lines @@
 main() {
   testFileUri();
   testFileUriWindowsSlash();
   testFileUriDriveLetter();
   testFileUriWindowsWin32Namespace();
   testFileUriResolve();
   testFileUriIllegalCharacters();
   testFileUriIllegalDriveLetter();
   testAdditionalComponents();
 }