Use List instead of Set for library sources.

pkg/analyzer/lib/src/task/dart.dart

 // Copyright (c) 2015, 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 analyzer.src.task.dart;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 3519 matching lines @@
  * The kind of the source closure to build.
  */
 enum _SourceClosureKind { IMPORT, EXPORT, IMPORT_EXPORT }
 
 /**
  * A [TaskInputBuilder] to build values for [_ImportSourceClosureTaskInput].
  */
 class _SourceClosureTaskInputBuilder implements TaskInputBuilder<List<Source>> {
   final _SourceClosureKind kind;
   final Set<LibraryElement> _libraries = new HashSet<LibraryElement>();
-  final Set<Source> _newSources = new HashSet<Source>();
+  final List<Source> _newSources = <Source>[];
 
   Source currentTarget;
 
   _SourceClosureTaskInputBuilder(Source librarySource, this.kind) {
     _newSources.add(librarySource);
   }
 
   @override
   ResultDescriptor get currentResult => LIBRARY_ELEMENT2;
 
@@ skipping 27 matching lines @@
   void currentValueNotAvailable() {
     // Nothing needs to be done.  moveNext() will simply go on to the next new
     // source.
   }
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
-    currentTarget = _newSources.first;
-    _newSources.remove(currentTarget);
+    currentTarget = _newSources.removeLast();
     return true;
   }
 }