Use relative URIs for core library part-of statements.

sdk/lib/core/errors.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";
 
 /**
  * Error objects thrown in the case of a program failure.
  *
  * An `Error` object represents a program failure that the programmer
  * should have avoided.
  *
  * Examples include calling a function with invalid arguments,
  * or even with the wrong number of arguments,
  * or calling it at a time when it is not allowed.
@@ skipping 543 matching lines @@
       : "Reading static variable '$variableName' during its initialization";
 }
 
 /// Used by Fasta to throw a compile-time error in a way that is compatible
 /// with compile-time constant evaluation.
 class _ConstantExpressionError {
   const _ConstantExpressionError();
 
   external _throw(error);
 }