Add support for user-defined HTML files.

lib/dart.js

+// Copyright (c) 2015, 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 runs in HTML files and loads the corresponding test scripts for
+// either Dartium or a JS browser. It's used by "pub serve" and user-authored
+// HTML files; when running without "pub serve", the default HTML file manually
+// chooses between serving a Dart or JS script tag.
+window.onload = function() {
+
+// Sends an error message to the server indicating that the script failed to
+// load.
+//
+// This mimics a MultiChannel-formatted message.
+var sendLoadException = function(message) {
+  window.parent.postMessage({
+    "href": window.location.href,
+    "data": [0, {"type": "loadException", "message": message}]
+  }, window.location.origin);
+}
+
+// The basename of the current page.
+var name = window.location.href.replace(/.*\//, '').replace(/#.*/, '');
+
+// Find <link rel="x-dart-test">.
+var links = document.getElementsByTagName("link");
+var testLinks = [];
+var length = links.length;
+for (var i = 0; i < length; ++i) {
+  if (links[i].rel == "x-dart-test") testLinks.push(links[i]);
+}
+
+if (links.length != 1) {
+  sendLoadException(
+      'Expected exactly 1 <link rel="x-dart-test"> in ' + name + ', found ' +
+          links.length + '.');
+  return;
+}
+
+var link = links[0];
+
+if (link.href == '') {
+  sendLoadException(
+      'Expected <link rel="x-dart-test"> in ' + name + ' to have an "href" ' +
+          'attribute.');
+  return;
+}
+
+var script = document.createElement('script');
+
+// Load the compiled JS for a normal browser, and the Dart code for Dartium.
+if (navigator.userAgent.indexOf('(Dart)') === -1) {
+  script.src = link.href + '.browser_test.dart.js';
+} else {
+  script.src = link.href + '.browser_test.dart';
+  script.type = 'application/dart';
+}
+
+script.onerror = function(event) {
+  var message = "Failed to load script at " + script.src +
+      (event.message ? ": " + event.message : ".");
+  sendLoadException(message);
+}
+
+var parent = link.parentNode;
+document.currentScript = script;
+parent.replaceChild(script, link);
+
+};