fix warning in polymer script_compactor

pkg/polymer/lib/src/build/script_compactor.dart

 // Copyright (c) 2013, 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.
 
 /** Transfomer that combines multiple dart script tags into a single one. */
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 24 matching lines @@
   /** Only run on entry point .html files. */
   Future<bool> isPrimary(Asset input) =>
       new Future.value(options.isHtmlEntryPoint(input.id));
 
   Future apply(Transform transform) {
     var id = transform.primaryInput.id;
     var secondaryId = id.addExtension('.scriptUrls');
     var logger = transform.logger;
     return readPrimaryAsHtml(transform).then((document) {
       return transform.readInputAsString(secondaryId).then((libraryIds) {
-        var libraries = JSON.decode(libraryIds).map(
+        var libraries = (JSON.decode(libraryIds) as Iterable).map(
           (data) => new AssetId.deserialize(data)).toList();
         var mainLibraryId;
         var mainScriptTag;
         bool changed = false;
 
         for (var tag in document.queryAll('script')) {
           var src = tag.attributes['src'];
           if (src == 'packages/polymer/boot.js') {
             tag.remove();
             continue;
@@ skipping 56 matching lines @@
       });
     });
   }
 }
 
 const MAIN_HEADER = """
 library app_bootstrap;
 
 import 'package:polymer/polymer.dart';
 """;