DEV: Upgrade our widget handlebars compiler

Now supports subexpressions such as i18n and concat, plus automatic
attaching of widgets similar to ember.

test/javascripts/widgets/widget-test.js.es6

@@ -204,6 +204,45 @@ widgetTest("widget attaching", {
   }
 });
 
+widgetTest("magic attaching by name", {
+  template: `{{mount-widget widget="attach-test"}}`,
+
+  beforeEach() {
+    createWidget("test-embedded", { tagName: "div.embedded" });
+
+    createWidget("attach-test", {
+      tagName: "div.container",
+      template: hbs`{{test-embedded attrs=attrs}}`
+    });
+  },
+
+  test(assert) {
+    assert.ok(find(".container").length, "renders container");
+    assert.ok(find(".container .embedded").length, "renders attached");
+  }
+});
+
+widgetTest("custom attrs to a magic attached widget", {
+  template: `{{mount-widget widget="attach-test"}}`,
+
+  beforeEach() {
+    createWidget("testing", {
+      tagName: "span.value",
+      template: hbs`{{attrs.value}}`
+    });
+
+    createWidget("attach-test", {
+      tagName: "div.container",
+      template: hbs`{{testing value=(concat "hello" " " "world")}}`
+    });
+  },
+
+  test(assert) {
+    assert.ok(find(".container").length, "renders container");
+    assert.equal(find(".container .value").text(), "hello world");
+  }
+});
+
 widgetTest("handlebars d-icon", {
   template: `{{mount-widget widget="hbs-icon-test" args=args}}`,