More refactoring of enqueuers

pkg/compiler/lib/src/universe/world_builder.dart

 // Copyright (c) 2012, 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.
 
 library universe;
 
 import 'dart:collection';
 
 import '../cache_strategy.dart';
 import '../common.dart';
@@ skipping 121 matching lines @@
 
   /// Returns `true` if [member] is invoked as a setter.
   bool hasInvokedSetter(Element member, World world);
 }
 
 abstract class ResolutionWorldBuilder implements WorldBuilder {
   /// Set of (live) local functions (closures) whose signatures reference type
   /// variables.
   ///
   /// A live function is one whose enclosing member function has been enqueued.
-  Set<Element> get closuresWithFreeTypeVariables;
+  Iterable<Element> get closuresWithFreeTypeVariables;
 
   /// Set of (live) `call` methods whose signatures reference type variables.
   ///
   /// A live `call` method is one whose enclosing class has been instantiated.
   Iterable<Element> get callMethodsWithFreeTypeVariables;
 
   /// Set of all closures in the program. Used by the mirror tracking system
   /// to find all live closure instances.
   Iterable<LocalFunctionElement> get allClosures;
 
@@ skipping 772 matching lines @@
 
   void forgetElement(Element element, Compiler compiler) {
     _directlyInstantiatedClasses.remove(element);
     if (element is ClassElement) {
       assert(invariant(element, element.thisType.isRaw,
           message: 'Generic classes not supported (${element.thisType}).'));
       _instantiatedTypes..remove(element.rawType)..remove(element.thisType);
     }
   }
 }