Merge the dart:uri library into dart:core and update the Uri class

tests/compiler/dart2js/uri_extras_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";
-import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/implementation/util/uri_extras.dart';
 
 
 void testRelativize() {
   void c(String expected, String base, String path, bool isWindows) {
     if (isWindows == null) {
       c(expected, base, path, true);
       c(expected, base, path, false);
       return;
@@ skipping 45 matching lines @@
 
   c('../person/file.dart', '/Users/other/', '/Users/person/file.dart', false);
 
   c('/Users/person/file.dart', '/Users/other/', '/Users/person/file.dart',
     true);
 }
 
 void main() {
   testRelativize();
 }