REGRESSION(r171966): Custom elements in async imports don't get upgrade.

LayoutTests/fast/html/imports/resources/import-custom-element-helper.js

+
+function registerTestingCustomElement(tagName) {
+    var definition = function() {};
+    definition.ids = [];
+    definition.prototype = Object.create(HTMLElement.prototype);
+    definition.prototype.createdCallback = function() { definition.ids.push(this.id); }
+    document.registerElement(tagName, definition);
+    return definition;

[dominicc (has gone to gerrit), 2014/04/25 02:39:45]

Why not just set up the prototype and return the generated constructor? You can
stick ids = [] on the generated constructor.

+}
+
+function ImportTestLatch(test, count) {
+    this.loaded = function() {
+        count--;
+        if (!count)
+            test();
+    };
+}