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

sdk/lib/collection/set.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Base implementations of [Set].
  */
-part of "dart:collection";
+part of dart.collection;
 
 /**
  * Mixin implementation of [Set].
  *
  * This class provides a base implementation of a `Set` that depends only
  * on the abstract members: [add], [contains], [lookup], [remove],
  * [iterator], [length] and [toSet].
  *
  * Some of the methods assume that `toSet` creates a modifiable set.
  * If using this mixin for an unmodifiable set,
@@ skipping 282 matching lines @@
 abstract class SetBase<E> extends SetMixin<E> {
   /**
    * Convert a `Set` to a string as `{each, element, as, string}`.
    *
    * Handles circular references where converting one of the elements
    * to a string ends up converting [set] to a string again.
    */
   static String setToString(Set set) =>
       IterableBase.iterableToFullString(set, '{', '}');
 }