Revert "Use relative URIs for core library part-of statements."

sdk/lib/math/rectangle.dart

Index: sdk/lib/math/rectangle.dart
diff --git a/sdk/lib/math/rectangle.dart b/sdk/lib/math/rectangle.dart
index d6a87528049e2fa5ee632abb5cb35aa7794eda2f..563713c4c108cc49830c6b3416a5b02ff718a82c 100644
--- a/sdk/lib/math/rectangle.dart
+++ b/sdk/lib/math/rectangle.dart
@@ -1,8 +1,7 @@
 // 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.
-
-part of "math.dart";
+part of dart.math;
 
 /**
  * A base class for representing two-dimensional axis-aligned rectangles.