Use relative URIs for core library part-of statements.

sdk/lib/core/print.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.
 
-part of dart.core;
+part of "core.dart";
 
 /// Prints a string representation of the object to the console.
 void print(Object object) {
   String line = "$object";
   if (printToZone == null) {
     printToConsole(line);
   } else {
     printToZone(line);
   }
 }