Use relative URIs for core library part-of statements.

sdk/lib/convert/base64.dart

 // Copyright (c) 2015, 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.convert;
+part of "convert.dart";
 
 /**
  * A [base64](https://tools.ietf.org/html/rfc4648) encoder and decoder.
  *
  * It encodes using the default base64 alphabet,
  * decodes using both the base64 and base64url alphabets,
  * does not allow invalid characters and requires padding.
  *
  * Examples:
  *
@@ skipping 846 matching lines @@
     end = RangeError.checkValidRange(start, end, string.length);
     if (start == end) return;
     Uint8List buffer = _decoder.decode(string, start, end);
     if (buffer != null) _sink.add(buffer);
     if (isLast) {
       _decoder.close(string, end);
       _sink.close();
     }
   }
 }