Implement custom element construction and some 'define' checks

third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

+function create_window() {
+  let p = new Promise((resolve) => {
+    let f = document.createElement('iframe');
+    f.srcdoc = '';
+    f.onload = (event) => {
+      let w = f.contentWindow;
+      w.tearDown = () => f.remove();
+      resolve(w);
+    };
+    document.body.appendChild(f);
+  });
+  return p;
+}
+
+function async_test_with_window(f, name) {

[domenic, 2016/05/05 19:15:28]

In general async_test is worse than promise_test; might want to create a
different variant. Seems unused in this CL though.

+  async_test((t) => {
+    create_window().then((w) => {
+      t.add_cleanup(() => w.tearDown());
+      t.step(() => f(t, w));
+    });
+  }, name);
+}
+
+function test_with_window(f, name) {
+  async_test_with_window((t, w) => {
+    f(w);
+    t.done();
+  }, name);
+}