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

sdk/lib/core/helpers.dart

Index: sdk/lib/core/helpers.dart
diff --git a/sdk/lib/uri/helpers.dart b/sdk/lib/core/helpers.dart
similarity index 97%
rename from sdk/lib/uri/helpers.dart
rename to sdk/lib/core/helpers.dart
index 2a16a47c7376c3116233fe64e5d12b99b807194a..cfbd2c834a037e7fe161ceb3fb280cd9193ecd5f 100644
--- a/sdk/lib/uri/helpers.dart
+++ b/sdk/lib/core/helpers.dart
@@ -2,7 +2,7 @@
 // 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.
 
-part of dart.uri;
+part of dart.core;
 
 String merge(String base, String reference) {
   if (base == "") return "/$reference";