Move IterableMixinWorkaround to internal library.

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

 // 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.
 
 // DO NOT EDIT - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:html library.
 
 /// The Dart HTML library.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
+import 'dart:_collection-dev';
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
 import 'dart:math';
 import 'dart:web_sql';
 // Not actually used, but imported since dart:html can generate these objects.
 import 'dart:svg' as svg;
 import 'dart:web_audio' as web_audio;
 import 'dart:web_gl' as gl;
@@ skipping 58 matching lines @@
   }
   return JS('int', r'window.$dart$isolate$counter++');
 }
 
 // Fast path to invoke JS send port.
 _callPortSync(int id, message) {
   return JS('var', r'ReceivePortSync.dispatchCall(#, #)', id, message);
 }
 
 spawnDomFunction(f) => IsolateNatives.spawnDomFunction(f);