[azakai]

SPDY does have some similarities, but the differences here are very large.

As I said, Dart runs differently in the VM and in dart2js. SPDY was just not present in other browsers at first. So there wasn't a case of sites breaking in subtle and hard to debug ways in SPDY, but there is in Dart. (Later, when other browsers started to support SPDY, there were the usual spec issues, but that's a separate matter.)

The second issue I raised was that someone could ship Dart code alone, without the JS fallback. In that case, it is indeed more similar to SPDY, as someone could in principle write a SPDY-only website. Even here there is a difference, though - SPDY relies on client+server interaction. Dart is client-only (there's a Dart server, but that's not relevant here). Client-only code can more easily stay on the web for a long time, possibly forgotten and unmaintained. But this might be a small difference, so I somewhat agree with you on this point, but disagree on the previous.