Move IterableMixinWorkaround to internal library.

sdk/lib/web_gl/dart2js/web_gl_dart2js.dart

 library dart.dom.web_gl;
 
 import 'dart:collection';
+import 'dart:_collection-dev';
 import 'dart:html';
 import 'dart:html_common';
 import 'dart:_js_helper' show Creates, JSName, Null, Returns, convertDartClosureToJS;
 import 'dart:_foreign_helper' show JS;
 // DO NOT EDIT - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:web_gl library.
 
 
 
@@ skipping 1966 matching lines @@
 }
 // 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.
 
 
 @DocsEditable
 @DomName('WebGLVertexArrayObjectOES')
 class VertexArrayObject native "*WebGLVertexArrayObjectOES" {
 }