fix bug where dart_support.js could get included above platform.js

pkg/polymer/lib/src/build/polyfill_injector.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.
 
 /// Includes any additional polyfills that may needed by the deployed app.
 library polymer.src.build.polyfill_injector;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 20 matching lines @@
     var id = idOrAsset is AssetId ? idOrAsset : idOrAsset.id;
     return new Future.value(options.isHtmlEntryPoint(id));
   }
 
   Future apply(Transform transform) {
     var logger = new BuildLogger(transform,
         convertErrorsToWarnings: !options.releaseMode,
         detailsUri: 'http://goo.gl/5HPeuP');
     return readPrimaryAsHtml(transform, logger).then((document) {
       bool dartSupportFound = false;
-      bool platformJsFound = false;
+      Element platformJs;
       Element dartJs;
       final dartScripts = <Element>[];
 
       for (var tag in document.querySelectorAll('script')) {
         var src = tag.attributes['src'];
         if (src != null) {
           var last = src.split('/').last;
           if (_platformJS.hasMatch(last)) {
-            platformJsFound = true;
+            platformJs = tag;
           } else if (_dartSupportJS.hasMatch(last)) {
             dartSupportFound = true;
           } else if (last == 'dart.js') {
             dartJs = tag;
           }
         }
 
         if (tag.attributes['type'] == 'application/dart') {
           dartScripts.add(tag);
         }
@@ skipping 29 matching lines @@
       } else {
         document.body.nodes.add(parseFragment(
               '<script src="packages/browser/dart.js"></script>'));
       }
 
       _addScriptFirst(urlSegment) {
         document.head.nodes.insert(0, parseFragment(
               '<script src="packages/$urlSegment"></script>\n'));
       }
 
-      var suffix = options.releaseMode ? '.js' : '.concat.js';
-      if (!dartSupportFound) _addScriptFirst('web_components/dart_support.js');
-      // platform.js should come before all other scripts.
-      if (!platformJsFound && options.injectPlatformJs) {
+      // Inserts dart_support.js either at the top of the document or directly
+      // after platform.js if it exists.
+      if (!dartSupportFound) {
+        if (platformJs == null) {
+          _addScriptFirst('web_components/dart_support.js');
+        } else {
+          var parentsNodes = platformJs.parentNode.nodes;
+          parentsNodes.insert(
+              parentsNodes.indexOf(platformJs) + 1,
+              parseFragment(
+                  '\n<script src="packages/web_components/dart_support.js">'
+                  '</script>'));
+        }
+      }
+
+      // By default platform.js should come before all other scripts.
+      if (platformJs == null && options.injectPlatformJs) {
+        var suffix = options.releaseMode ? '.js' : '.concat.js';
         _addScriptFirst('web_components/platform$suffix');
       }
 
       transform.addOutput(
           new Asset.fromString(transform.primaryInput.id, document.outerHtml));
     });
   }
 }
 
 final _platformJS = new RegExp(r'platform.*\.js', caseSensitive: false);
 final _dartSupportJS = new RegExp(r'dart_support.js', caseSensitive: false);