Drop support for legacy iframe communication

[natebosch]

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.

[jakemac53]

Are we going to run into the issue with missing flutter_test dependencies again?

[natebosch]

Are we going to run into the issue with missing flutter_test dependencies again?

We shouldn't. The two sides of this particular dependency are managed by dependencies in the flutter checkout, not the users project.

flutter/flutter#131881 updated flutter_tools to send the 'port' message, and the compiled host.dart.js is used from the flutter repo pub solve.
https://github.com/flutter/flutter/blob/4d8e390c1be6237ab45df9c57b87e50e499056db/packages/flutter_tools/lib/src/test/flutter_web_platform.dart#L156-L157

This would only be a problem if the iframe code could be an old version - but that is controlled by the test platform, not the pub dependency.