Drop support for legacy iframe communication

Closes #2065

All known iframe side implementations have been updated to send the
`'port'` message with an immediately available channel over the map and
the extra window message.

Move some still relevant `assert` into the implementation for the
current approach.

pkgs/test/tool/host.dart

@@ -231,36 +231,15 @@ StreamChannel<dynamic> _connectToIframe(String url, int id) {
         dom.window.console.log('Connecting channel for suite $suiteUrl');
         // The frame is starting and sending a port to forward for the suite.
         final port = message.ports.first;
+        assert(!_domSubscriptions.containsKey(id));
         _domSubscriptions[id] =
             dom.Subscription(port, 'message', allowInterop((event) {
           controller.local.sink.add((event as dom.MessageEvent).data);
         }));
         port.start();
 
-        _subscriptions[id] = controller.local.stream.listen(port.postMessage);
-      case {'ready': true}:
-        // This message indicates that the iframe is actively listening for
-        // events, so the message channel's second port can now be transferred.
-        var channel = dom.createMessageChannel();
-        assert(!_domSubscriptions.containsKey(id));
-        _domSubscriptions[id] = dom.Subscription(channel.port1, 'message',
-            allowInterop((dom.Event event) {
-          controller.local.sink.add((event as dom.MessageEvent).data['data']);
-        }));
-
         assert(!_subscriptions.containsKey(id));
-        _subscriptions[id] =
-            controller.local.stream.listen(channel.port1.postMessage);
-        channel
-          ..port2.start()
-          ..port1.start();
-        // TODO(#1758): This is a work around for a crash in package:build.
-        js_util.callMethod(
-            js_util.getProperty(iframe, 'contentWindow'), 'postMessage', [
-          'port',
-          dom.window.location.origin,
-          [channel.port2]
-        ]);
+        _subscriptions[id] = controller.local.stream.listen(port.postMessage);
       case {'exception': true, 'data': final data}:
         // This message from `dart.js` indicates that an exception occurred
         // loading the test.