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

sdk/lib/core/string_buffer.dart

 // Copyright (c) 2011, 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 dart.core;
 
 /**
  * A class for concatenating strings efficiently.
  *
  * Allows for the incremental building of a string using write*() methods.
  * The strings are concatenated to a single string only when [toString] is
  * called.
  */
 class StringBuffer implements StringSink {
   /** Creates the string buffer with an initial content. */
@@ skipping 25 matching lines @@
   external void writeln([Object obj = ""]);
 
   /**
    * Clears the string buffer.
    */
   external void clear();
 
   /// Returns the contents of buffer as a concatenated string.
   external String toString();
 }