Added a Test for 2 way communication

tests/fixtures/two-way-iframe.html

@@ -0,0 +1,10 @@
+<script type="module">
+  import * as Comlink from "/base/dist/esm/comlink.mjs";
+
+  const parentEndpoint = Comlink.windowEndpoint(self.parent);
+  const wrappedParent = Comlink.wrap(parentEndpoint);
+
+  Comlink.expose(async (a, b) => {
+    return a + (await wrappedParent(b));
+  }, parentEndpoint);
+</script>

tests/two-way-iframe.comlink.test.js

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+/**
+ * Copyright 2017 Google Inc. All Rights Reserved.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as Comlink from "/base/dist/esm/comlink.mjs";
+
+describe("Comlink across iframes", function () {
+  beforeEach(function () {
+    this.ifr = document.createElement("iframe");
+    this.ifr.sandbox.add("allow-scripts", "allow-same-origin");
+    this.ifr.src = "/base/tests/fixtures/two-way-iframe.html";
+    document.body.appendChild(this.ifr);
+    return new Promise((resolve) => (this.ifr.onload = resolve));
+  });
+
+  afterEach(function () {
+    this.ifr.remove();
+  });
+
+  it("can communicate both ways", async function () {
+    let called = false;
+    const iframe = Comlink.windowEndpoint(this.ifr.contentWindow);
+    Comlink.expose((a) => {
+      called = true;
+      return ++a;
+    }, iframe);
+    const proxy = Comlink.wrap(iframe);
+    expect(await proxy(1, 3)).to.equal(5);
+    expect(called).to.equal(true);
+  });
+});