Revert "Rename internal library dart:_collection-dev to dart:_internal."

sdk/lib/internal/iterable.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._collection.dev;
+part of dart._internal;
 
 /**
  * Marker interface for [Iterable] subclasses that have an efficient
  * [length] implementation.
  */
 abstract class EfficientLength {
   /**
    * Returns the number of elements in the iterable.
    *
    * This is an efficient operation that doesn't require iterating through
@@ skipping 1142 matching lines @@
 
   static Set setDifference(Set set, Set other, Set result) {
     for (var element in set) {
       if (!other.contains(element)) {
         result.add(element);
       }
     }
     return result;
   }
 }