Show error message if dart.js is also together with boot.js on the page.

pkg/polymer/lib/boot.js

 // 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.
 
 // This script dynamically prepares a set of files to run polymer.dart. It uses
 // the html_import polyfill to search for all imported files, then
 // it inlines all <polymer-element> definitions on the top-level page (needed by
 // registerPolymerElement), and it removes script tags that appear inside
 // those tags. It finally rewrites the main entrypoint to call an initialization
 // function on each of the declared <polymer-elements>.
@@ skipping 39 matching lines @@
 (function() {
   // Only run in Dartium.
   if (!navigator.webkitStartDart) {
     // TODO(sigmund): rephrase when we split build.dart in two: analysis vs
     // deploy pieces.
     console.warn('boot.js only works in Dartium. Run the build.dart' +
       ' tool to compile a depolyable JavaScript version')
     return;
   }
 
+  // Detect if dart.js was included in the page. We use DOMContentLoaded to make
+  // sure we also check for occurrences after the current tag.
+  window.addEventListener('DOMContentLoaded', function () {
+    var dartJSQuery = 'script[src="packages/browser/dart.js"]';
+    if (!!document.querySelector(dartJSQuery)) {
+      console.error('Error: found "packages/browser/dart.js" in the ' +
+        'page. Please remove it. When using polymer\'s "boot.js", ' +
+        'you can\'t use "dart.js".');
+    }
+  });
 
   // Load HTML Imports:
   var htmlImportsSrc = 'src="packages/html_import/html_import.min.js"';
   document.write('<script ' + htmlImportsSrc + '></script>');
   var importScript = document.querySelector('script[' + htmlImportsSrc + ']');
   importScript.addEventListener('load', function() {
     // NOTE: this is from polymer/src/lib/dom.js
     window.HTMLImports.importer.preloadSelectors +=
         ', polymer-element link[rel=stylesheet]';
   });
@@ skipping 105 matching lines @@
     alreadyRan = true;
     var ref = document.body.children[0];
     inlinePolymerElements(document, ref);
     mergeScripts();
     if (!navigator.webkitStartDart()) {
       document.body.innerHTML = 'This build has expired. Please download a ' +
           'new Dartium at http://www.dartlang.org/dartium/index.html';
     }
   });
 })();