Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/util/uri_extras.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.
 
 library uri_extras;
 
 import 'dart:math';
 
 String relativize(Uri base, Uri uri, bool isWindows) {
   if (!base.path.startsWith('/')) {
@@ skipping 48 matching lines @@
       sb.write('../');
     }
     for (int i = common; i < uriParts.length - 1; i++) {
       sb.write('${uriParts[i]}/');
     }
     sb.write('${uriParts.last}');
     return sb.toString();
   }
   return uri.toString();
 }