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

sdk/lib/internal/symbol.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:_internal";
+part of dart._internal;
 
 /**
  * Implementation of [core.Symbol].  This class uses the same name as
  * a core class so a user can't tell the difference.
  *
  * The purpose of this class is to hide [_name] from user code, but
  * make it accessible to Dart platform code via the static method
  * [getName].
  */
 class Symbol implements core.Symbol {
@@ skipping 115 matching lines @@
 
   /**
    * Checks whether name is a valid symbol name.
    *
    * This test allows both private and non-private symbols.
    */
   static bool isValidSymbol(String name) {
     return (name.isEmpty || symbolPattern.hasMatch(name));
   }
 }