Revert "Change some platform libraries to use `part of URI`."

sdk/lib/convert/ascii.dart

 // 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 "dart:convert";
+part of dart.convert;
 
 /**
  * An instance of the default implementation of the [AsciiCodec].
  *
  * This instance provides a convenient access to the most common ASCII
  * use cases.
  *
  * Examples:
  *
  *     var encoded = ASCII.encode("This is ASCII!");
@@ skipping 285 matching lines @@
     RangeError.checkValidRange(start, end, length);
     if (start < end) {
       if (start != 0 || end != length) {
         source = source.sublist(start, end);
       }
       add(source);
     }
     if (isLast) close();
   }
 }