Convert mediaCast sender to typescript

ext/src/shim/export.ts

@@ -0,0 +1,34 @@
+"use strict";
+
+import * as cast from "./cast";
+
+import { BridgeInfo } from "../lib/getBridgeInfo";
+import { Message } from "../types";
+import { onMessage } from "./messageBridge";
+
+
+/**
+ * To support exporting an API from a module, we need to
+ * retain the event-based message passing despite not
+ * actually crossing any context boundaries. The shim listens
+ * for and emits these messages, and changing that behavior
+ * is too messy.
+ */
+export function init (): Promise<BridgeInfo> {
+    return new Promise(async (resolve, reject) => {
+
+        // Trigger message port setup side-effects
+        import("./content");
+
+        onMessage(message => {
+            switch (message.subject) {
+                case "shim:/initialized": {
+                    const bridgeInfo: BridgeInfo = message.data;
+                    resolve(bridgeInfo);
+                }
+            }
+        })
+    });
+}
+
+export default cast;