Send a MessagePort to host on frame startup

[natebosch]

Towards #2065

All host implementations are updated to allow either the legacy format,
or this new format.

Simplify the communication and avoid a listener on the frame window
message events. Upon startup replace the {'ready': true} message with
a 'port' message and the MessagePort for a channel created on the
frame side. Omit the manual 'href' field, no host implementations would
read it.

[jakemac53]

Can you check this with package:build (or by just passing -O2 to dart2js in this package)?

[natebosch]

Can you check this with package:build (or by just passing -O2 to dart2js in this package)?

I recompiled host.js with -O2 and dart test -p chrome still works. I also overrode the dep to this version in build/_test and ran dart run build_runner test -- -p chrome and that worked too.